Osisko Development Intercepts 57.29 G/T Gold over 3.05 Meters, 625.95 G/T Gold over 0.5 Meters and 15.26 G/T Gold over 4.4 Meters in Lowhee Infill Drilling at Cariboo Gold Project

Chris Lodder, President, stated, "The recent underground infill drilling at Lowhee Zone was conducted to analyze how grade holds up under tighter drilling and to investigate optimal drill hole spacing for future mining. We’re highly encouraged that underground exploration drilling in the Lowhee Zone and across the deposit completed to date continues to indicate that veins remain open at depth, highlighting the strong potential for future resource expansion through additional exploration programs leveraging existing underground infrastructure. A comprehensive 13,000-meter infill drill program is already underway, designed to deliver important data for local resource modeling, mine planning, and optimization of production stope design. We anticipate providing regular progress updates on this program in the coming months."

DRILL ASSAY HIGHLIGHTS

This news release includes assays from sixty-eight (68) underground infill and exploration diamond drill ("DD") holes totaling approximately 6,471 m completed from November 2024 through early August 2025 (see Table 1) and for which assays were received by a cut off date of August 16, 2025. Select fire and photon assay highlights include:

• 95 grams per tonne ("g/t") gold ("Au") over 0.5 m in hole BMU-25-040
• 29 g/t Au over 3.05 m in hole BMU-25-032, including:
• 20 g/t Au over 0.5 m
• 57 g/t Au over 0.6 m
• 77 g/t Au over 1.0 m
• 26 g/t Au over 4.4 m in hole BMU-25-004, including:
• 40 g/t au over 0.5 m
• 80 g/t Au over 0.5 m
• 23 g/t Au over 2.4 m in hole BMU-25-025 including:
• 96 g/t Au over 0.8 m
• 45 g/t Au over 0.55 m
• 39 g/t Au over 0.5 m
• 74 g/t Au over 1.0 m in hole BMU-25-034, including:
• 15 g/t Au over 0.5 m
• 34 g/t Au over 0.5 m
• 37 g/t Au over 5.9 m in hole BMU-25-014, including:
• 24 g/t Au over 0.5 m
• 97 g/t Au over 0.5 m
• 02 g/t Au over 0.55 m
• 88 g/t Au over 0.5 m
• 41 g/t Au over 0.5 m
• 74 g/t Au over 0.5 m
• 06 g/t Au over 6.4 m in hole BMU-25-026, including:
• 61 g/t Au over 0.5 m
• 53 g/t Au over 0.5 m
• 44 g/t Au over 3.45 m in hole BMU-25-017, including:
• 17 g/t Au over 0.5 m
• 93 g/t Au over 0.5 m
• 23 g/t Au over 5.75 m in hole BMU-25-019, including:
• 18 g/t Au over 0.5 m
• 32 g/t Au over 0.5 m
• 31 g/t Au over 0.5 m
• 55 g/t Au over 1.45 m in hole BMU-25-045, including:
• 20 g/t Au over 0.5 m

True widths are estimated to be between 50% and 90% of reported core length intervals. Intervals not recovered by drilling were assigned zero grade. Top cuts have not been applied to high grade assays. Complete assay highlights are presented in Table 1 and drill hole locations and orientations are listed in Table 3.

INFILL AND EXPLORATION PROGRAM SUMMARY

• 14 NQ (47.6-millimeter diameter) diamond drill holes (BMU-25-008 to 021) with depth ranging from 42 to 48 m focused on infilling a 25 m long section of planned ore drive.
• 4 NQ diamond drill holes (BMU-25-001, 003, 004, 007) with an average depth of 290 m were drilled to explore zones adjacent to the existing Lowhee Zone mineral resources.
• 22 NQ diamond drill holes (BMU-25-032 to 053) with an average depth of 90 m conducting systematic grid infill on a set of mineral reserve blocks, mineral inferred resources, and non-DSO resources.
• 8 NQ diamond drill holes (BMU-24-001 to 008) with an average depth of 118 m were drilled infilling areas within existing mineral reserves and resources.

NEXT STEPS

• A 13,000-meter systematic grid infill program is underway within the Lowhee Zone targeting mineral reserve blocks, inferred mineral resources, and non-DSO resources in an approximate 100 m swath transecting approximately the full width of the Lowhee Zone (Figure 3). Drilling at 10 m centers from new development at the 1,260 elevation level, the program is expected to provide a comprehensive data set that will inform resource modeling, mine planning, production stope design procedures and parameters, and the appropriate grid spacing for future infill drilling. Additional systematic grid infill drill programs are planned in step with future underground development progress into other areas of the deposit.
• Structural modelling suggests that the sandstone rock package, the primary host rock for the Cariboo deposit, remains open well beyond depths tested to date. Positive results from deeper drilling across the deposit highlight strong potential for extensions at depth, below the currently defined mineral reserves and resources (Figure 4).

UNDERGROUND CHIP/CHANNEL SAMPLING HIGHLIGHTS

This news release contains assays from eighty-nine (89) underground face and wall rock chip and rock saw channels ("CH") totaling approximately 398 m completed in ore and access development from October 2024 through July 2025 (see Table 2) and for which assays were received by a cut off date of July 31, 2025. Select fire and photon assay highlights include:

• 01 g/t Au over 3.8 m in chip channel UGCH00189, including:
• 37 g/t Au over 0.8 m
• 23 g/t Au over 4.6 m in chip channel UGCH00183, including:
• 95 g/t Au over 0.8 m
• 25 g/t Au over 4.2 m in rock saw channel UGCH00236, including:
• 64 g/t Au over 1.0 m
• 62 g/t Au over 0.9 m
• 81 g/t Au over 4.2 m in face chip channel UGCH00193, including:
• 66 g/t Au over 1.1 m
• 32 g/t Au over 6.5 m in face chip channel UGCH00213, including:
• 52 g/t Au over 1.0 m
• 77 g/t Au over 1.0 m
• 82 g/t Au over 4.2 m in rock saw channel UGCH00246, including:
• 22 g/t Au over 0.8 m
• 22 g/t Au over 4.2 m in rock saw channel UGCH00245, including:
• 51 g/t Au over 1.2 m
• 86 g/t Au over 1.5 m
• 61 g/t Au over 4.4 m in chip channel UGCH00176, including:
• 20 g/t Au over 0.7 m
• 02 g/t Au over 5.2 m in chip channel UGCH00210, including:
• 87 g/t Au over 1.0 m
• 13 g/t Au over 1.2 m
• 14 g/t Au over 3.7 m in rock saw channel UGCH00243, including:
• 65 g/t Au over 1.0 m
• 07 g/t Au over 1.2 m

True widths are estimated to be between 85% and 100% of reported sample intervals for all face sampling in ore drives and wall sampling in crosscuts. Wall samples in ore drives (along the vein) are excluded from the above list and indicated as select (Table 4). Top cuts have not been applied to high grade assays. Complete assay highlights are presented in Table 2 and sample locations are listed in Table 4.

ABOUT LOWHEE ZONE

Geological mapping and geochemical sampling were carried out on Barkerville Mountain from 2017-2018, with the Lowhee Zone identified as a high-priority drill target.

In 2019, two southeast-oriented stratigraphic and 22 northwest-southeast oriented drillholes (8,337.0 m) were drilled at the Lowhee Zone. The drilling successfully identified auriferous quartz-carbonate veins at similar orientations to those observed elsewhere on the Cariboo Gold project. Initial 3D geological modelling and resource estimation commenced, and further drilling was recommended.

In 2020, 24 northwest-oriented diamond drillholes (10,144.5 m) were drilled. The focus of the exploration program was to test the extent of mineralization along the down-dip and northeast strike-extent of veining. An internal resource estimation of the Lowhee deposit was completed. Further drilling was recommended to grow the confidence of the resource.

In 2021, a total of 94 diamond drillholes (29,449.1 m) were drilled. The focus of drilling was to delineate, and infill modelled veins with 25 m spacing. BGM’s resource modelling team produced a mineral resource estimate, and the collection of a bulk sample was recommended.

In 2022, a total of 27 diamond drillholes (6,563.90 m) were drilled. There were two main goals with this drill program. The first goal was to infill a potential bulk sample location achieving category conversion from indicated (25 m spacing) to measured (12.5 m spacing). The second goal was to continue to delineate and infill modelled veins with 25 m spacing.

Lowhee Zone access is through Cow portal on the northwestern flank of Barkerville Mountain (Figure 1 and Figure 3) Cow portal construction was completed in Q4 2024 and development of the underground ramp into the Lowhee zone commenced in Q1 2025. Approximately 350 m of development has been advanced within the Lowhee Zone deposit at the 1,290 and 1,260-elevation levels since completion of the main access ramp. The probable mineral reserves estimate for the Lowhee Zone includes 104,491 ounces of contained Au (923,162 tonnes grading 3.52 g/t Au) and represents approximately 5% of the total contained gold in the estimated probable mineral reserves for the Cariboo Gold Project.

ABOUT CARIBOO GOLD PROJECT

The Cariboo Gold Project is a permitted, 100%-owned feasibility-stage project located in the historic Wells-Barkerville mining camp of central British Columbia, Canada. Spanning approximately 186,740 hectares, the Company’s land package includes 443 mineral titles and covers an approximate 77-kilometre strike of highly prospective exploration targets extending northwest to southeast. In late 2024, the Project was granted the Mines Act and Environmental Management Act (British Columbia) permits, marking the successful completion of the permitting process for key approvals, solidifying the Project’s shovel-ready status.

The Cariboo Gold Project hosts probable mineral reserves of 2.07 million ounces of contained Au (17,815 kt grading 3.62 g/t Au); measured mineral resources of 8,000 ounces of contained Au (47 kt grading 5.06 g/t Au); indicated mineral resources of 1.60 million ounces of contained Au (17,332 kt grading 2.88 g/t Au); and inferred mineral resources of 1.86 million ounces of contained Au (18,774 kt grading 3.09 g/t Au). Mineral resources are reported exclusive of mineral reserves.

Technical Reports

Information relating to the Cariboo Gold Project and the 2025 FS on the Cariboo Gold Project is supported by the technical report, titled "NI 43-101 Technical Report, Feasibility Study for the Cariboo Gold Project, District of Wells, British Columbia, Canada" and dated June 11, 2025 (with an effective date of April 25, 2025) (the "Cariboo Technical Report").

For readers to fully understand the information in the Cariboo Technical Report, reference should be made to the full text of the Cariboo Technical Report, including all assumptions, parameters, qualifications, limitations and methods therein. The Cariboo Technical Report is intended to be read as in its entirety, and sections should not be read or relied upon out of context. The Cariboo Technical Report was prepared in accordance with NI 43-101 and is available electronically on SEDAR+ (www.sedarplus.ca) and on EDGAR (www.sec.gov) under Osisko Development’s issuer profile and on the Company’s website at www.osiskodev.com.

Qualified Persons

The scientific and technical information contained in this news release has been reviewed and approved by Eryn Doyle, P. Geo., Senior Exploration Manager of Osisko Development, a "qualified person" within the meaning of National Instrument 43-101 – Standards of Disclosure for Mineral Projects ("NI 43-101").

Quality Assurance (QA) – Quality Control (QC)

All drill core samples, once received from the drill and processed, are sawn in half, labelled and bagged. The remaining drill core is subsequently stored on site at a secured facility in Wells, B.C., Canada, with the exception of 615 m of core drilled from the L1260 Sump for which the second half of the drill core was also bagged and sent to the lab for analysis. This extra data was logged and stored as FIELD DUP samples. Numbered security tags are applied to lab shipments for chain of custody requirements. All material retrieved in chip and channel sampling is bagged for assay. Quality control (QC) samples are inserted at regular intervals in the sample stream for both diamond drilling and underground chip and channel sampling, including blanks and reference materials with all sample shipments to monitor laboratory performance.

Drill core and underground chip and channel samples are submitted to MSALABS’s analytical facility in Prince George, B.C., Canada, for preparation and analysis. The MSALABS facility is accredited to the ISO/IEC 17025 standard for gold assays and all analytical methods include quality control materials at set frequencies with established data acceptance criteria. The entire sample is dried, crushed, and split into sealed containers. Analysis for gold is by gamma ray analysis using the Chrysos Photon Assay (PA1408X). Samples are bombarded with gamma rays and the resulting signal is sent to the detectors.

Alternatively, drill core samples are submitted to ALS Geochemistry’s analytical facility in North Vancouver, British Columbia for preparation and analysis. The ALS facility is accredited to the ISO/IEC 17025 standard for gold assays and all analytical methods include quality control materials at set frequencies with established data acceptance criteria. The entire sample is crushed, and 250 grams is pulverized. Analysis for gold is by 50g fire assay fusion with atomic absorption (AAS) finish with a lower limit of 0.01 ppm and upper limit of 100 ppm. Samples with gold assays greater than 100 ppm are re-analyzed using a 1,000-gram screen metallic fire assay. A selected number of samples are also analyzed using a 48 multi-elemental geochemical package by a 4-acid digestion, followed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS).

Early program underground chip samples (UGCH00168 – UGCH00187) were transported to the Company’s nearby QR Mill Site complex for preparation and analysis at the internal QR Laboratory facilities. The Company’s QR Mill Laboratory is not a certified analytical laboratory, but the facility is managed by a qualified Chief Assayer and certified OREAS QC standards and blanks are inserted at regular intervals in the sample stream to monitor laboratory performance. Samples are dried, crushed to 2.0-2.4 mm and a 200 g split is taken. The split is pulverized, and a 20 g Fire Assay with Atomic Absorption Spectroscopy (AAS) finish is completed to determine gold grades, reported in g/t. All samples herein that were initially processed at the QR Mill Laboratory were subsequently sent to ALS Geochemistry’s North Vancouver facility for reanalysis with higher grade (> 3.0 g/t) run to extinction for more robust validation.

ABOUT OSISKO DEVELOPMENT CORP.

Osisko Development Corp. is a continental North American gold development company focused on past-producing mining camps located in mining friendly jurisdictions with district scale potential. The Company’s objective is to become an intermediate gold producer by advancing its flagship permitted 100%-owned Cariboo Gold Project, located in central B.C., Canada. Its project pipeline is complemented by the Tintic Project in the historic East Tintic mining district in Utah, U.S.A., and the San Antonio Gold Project in Sonora, Mexico—brownfield properties with significant exploration potential, extensive historical mining data, access to existing infrastructure and skilled labour. The Company’s strategy is to develop attractive, long-life, socially and environmentally responsible mining assets, while minimizing exposure to development risk and growing mineral resources.

For further information, visit our website at www.osiskodev.com or contact:

Sean Roosen

Philip Rabenok

Chairman and CEO

Vice President, Investor Relations

Email: sroosen@osiskodev.com

Email: prabenok@osiskodev.com

Tel: +1 (514) 940-0685

In Europe

Swiss Resource Capital AG

Jochen Staiger & Marc Ollinger

info@resource-capital.ch

www.resource-capital.ch

Tel: +1 (437) 423-3644

CAUTION REGARDING FORWARD LOOKING STATEMENTS

Certain statements contained in this news release may be deemed "forward-looking statements" within the meaning of the United States Private Securities Litigation Reform Act of 1995 and "forward-looking information" within the meaning of applicable Canadian securities legislation (together, "forward-looking statements"). These forward-looking statements, by their nature, require Osisko Development to make certain assumptions and necessarily involve known and unknown risks and uncertainties that could cause actual results to differ materially from those expressed or implied in these forward-looking statements. Forward-looking statements are not guarantees of performance. Words such as "may", "will", "would", "could", "expect", "believe", "plan", "anticipate", "intend", "estimate", "continue", "objective", "strategy", or the negative or comparable terminology, as well as terms usually used in the future and the conditional, are intended to identify forward-looking statements. Information contained in forward-looking statements is based upon certain material assumptions that were applied in drawing a conclusion or making a forecast or projection, including the assumptions, qualifications, limitations or statements relating to the prospectivity of exploration in the Lowhee Zone and targets outside of the currently defined mineral resources; the results (if any) of further exploration work to define and expand mineral resources at depth; the results, timing, utility and significance of the ongoing 13,000-meter infill drill program (if any); the ability and utility of exploration work (including drilling) to inform resource modeling, mine planning, production stope design procedures and parameters, and the appropriate grid spacing for future infill drilling (if at all); the ability and timing (if at all) to complete future additional systemic grid infill drill programs; the interpretation and accuracy of structure modeling and assumptions in regard to potential resource extensions at depth (if at all); the Company’s strategy and objectives relating to the Cariboo Gold Project as well as its other projects; the assumptions, qualifications and limitations relating to the Cariboo Gold Project being permitted; assumptions, qualifications and parameters underlying the Cariboo Technical Report (including, but not limited to, the mineral resources, mineral reserves, production profile, mine design and project economics); the results of the Cariboo Technical Report as an indicator of quality and robustness of the Cariboo Gold Project, as well as other considerations that are believed to be appropriate in the circumstances; the ability of the Company to achieve the estimates outlined in the Cariboo Technical Report in the timing contemplated (if at all); the ability to achieve the capital and operating costs outlined in the Cariboo Technical Report (if at all); the ability, progress and timing in respect of pre-construction activities at Cariboo including the 13,000-meter infill drill program; the contemplated work plan and activities at the Cariboo Gold Project and the timing, scope and results thereof and associated costs thereto; the potential impact of tariffs and other trade restrictions (if any); mineral resource category conversion; the future development and operations at the Cariboo Gold Project; management’s perceptions of historical trends, current conditions and expected future developments; the utility and significance of historic data, including the significance of the district hosting past producing mines; future mining activities; the results (if any) of further exploration work to define and expand mineral resources; the ability of exploration work (including drilling and sampling) to accurately predict mineralization; the ability of the Company to expand mineral resources beyond current mineral resource estimates; the ability of the Company to complete its exploration and development objectives for its projects in the timing contemplated and within expected costs (if at all); the ongoing advancement of the deposits on the Company’s properties; sustainability and environmental impacts of operations at the Company’s properties; gold prices; the Project being shovel-ready; the costs required to advance the Company’s properties; the ability to adapt to changes in gold prices, estimates of costs, estimates of planned exploration and development expenditures; the profitability (if at all) of the Company’s operations; regulatory framework remaining defined and understood as well as other considerations that are believed to be appropriate in the circumstances, and any other information herein that is not a historical fact may be "forward looking information". Osisko Development considers its assumptions to be reasonable based on information currently available, but cautions the reader that their assumptions regarding future events, many of which are beyond the control of Osisko Development, may ultimately prove to be incorrect since they are subject to risks and uncertainties that affect Osisko Development and its business. Such risks and uncertainties include, among others, risks relating to third-party approvals, including the issuance of permits by governments, capital market conditions and the Company’s ability to access capital on terms acceptable to the Company for the contemplated exploration and development at the Company’s properties; the ability to continue current operations and exploration; regulatory framework and presence of laws and regulations that may impose restrictions on mining; errors in management’s geological modelling; the timing and ability of the Company to obtain and maintain required approvals and permits; the results of exploration activities; risks relating to exploration, development and mining activities; the global economic climate; fluctuations in metal and commodity prices; fluctuations in the currency markets; dilution; environmental risks; and community, non-governmental and governmental actions and the impact of stakeholder actions. Osisko Development is confident a robust consultation process was followed in relation to its received BC Mines Act and Environmental Management Act permits for the Cariboo Gold Project and continues to actively consult and engage with Indigenous nations and stakeholders. While any party may seek to have the decision related to the BC Mines Act and/or Environmental Management Act permits reviewed by the courts, the Company does not expect that such a review would, were it to occur, impact its ability to proceed with the construction and operation of the Cariboo Gold Project in accordance with the approved BC Mines Act and Environmental Management Act permits. Readers are urged to consult the disclosure provided under the heading "Risk Factors" in the Company’s annual information form for the year ended December 31, 2024 as well as the financial statements and MD&A for the year ended December 31, 2024 and quarter ended June 30, 2025, which have been filed on SEDAR+ (www.sedarplus.ca) under Osisko Development’s issuer profile and on the SEC’s EDGAR website (www.sec.gov), for further information regarding the risks and other factors facing the Company, its business and operations. Although the Company’s believes the expectations conveyed by the forward-looking statements are reasonable based on information available as of the date hereof, no assurances can be given as to future results, levels of activity and achievements. The Company disclaims any obligation to update any forward-looking statements, whether as a result of new information, future events or results or otherwise, except as required by law. Forward-looking statements are not guarantees of performance and there can be no assurance that these forward-looking statements will prove to be accurate, as actual results and future events could differ materially from those anticipated in such statements. Accordingly, readers should not place undue reliance on forward-looking statements.

Neither the TSX Venture Exchange nor its Regulation Services Provider (as that term is defined in the policies of the TSX Venture Exchange) accepts responsibility for the adequacy or accuracy of this news release. No stock exchange, securities commission or other regulatory authority has approved or disapproved the information contained herein.