HIGHLIGHTS

• 8,971 m of new surface exploration drilling designed to test near mine potential at depth and in under-drilled gap zones between defined deposit footprints. Approximately 65% of drilling transected areas outside the Cariboo Gold deposit.
• Highlight intercepts from outside the footprint of the current mineral resource across multiple zones on strike, include: 13.19 g/t Au over 3.60 m at 596 m vertical depth, 6.40 g/t Au over 6.30 m at 326 m vertical depth (including 40.60 g/t Au over 0.50 m, 19.10 g/t Au over 0.70 m, and 4.56 g/t Au over 0.70 m), 6.17 g/t Au over 7.10 m at 517 m vertical depth (including 73.20 g/t Au over 0.50 m, 10.40 g/t Au over 0.50 m, and 2.21 g/t Au over 0.50 m) and 28.90 g/t Au over 0.50 m at 718 m vertical depth.
• Results to date confirm the presence of high-grade mineralization at depth and reinforce the strong potential for upside within under-tested gap zones and below the current Cariboo Gold deposit footprint, which extends to an average depth of approximately 350 metres.
• 5 drill rigs continue operating at surface systematically testing targets down-dip of the current Mineral Resource to a maximum vertical depth of up to 800 metres.

Osisko Development Corp. (NYSE: ODV, TSX-V: ODV) is pleased to announce new drilling results from eleven (11) surface diamond drill holes that were completed as part of an ongoing deeps and near mine gap infill exploration program at the Company’s permitted, 100%-owned Cariboo Gold Project, located in central British Columbia, Canada.

Chris Lodder, President, stated, “We are encouraged by the initial results of the surface exploration drill program at the Cariboo Gold Project, which intercepted mineralization in all eleven drill holes. Strong high-grade intercepts in previously untested areas and known under-tested gaps continue to demonstrate the at depth potential for resource growth beyond our current mineral inventory. These results reinforce our view that we are only beginning to define the full extent of the mineralized system at depth, while identifying compelling near-mine upside opportunities — all within the Project’s existing permitted footprint. We are at an early stage of this program and are excited to keep drilling.”

DRILL ASSAY HIGHLIGHTS

This news release includes assays from eleven surface deeps and gap zone exploration diamond drill holes totaling 8,971 metres with depths ranging from 114 to 1,159 m (see Table 1 and Figure 2) completed between December 2025 and April 2026. Approximately 65%, or 5,823 m of drilling transected areas outside the current Cariboo Gold Mineral Resources, as defined by a 15 m buffer around optimized Mineral Reserves and Mineral Resources stope shapes¹. All holes were collared in HQ (63.5 millimetre diameter) and reduced to NQ (47.6 millimetre diameter) where necessary to continue advance or implement directional drilling. Available tooling for directional drilling requires advance in NQ with a BQ (36.5 millimetre diameter) output while steering is active. All assays were received by June 1, 2026. Select assay highlights from intercepts outside the current Mineral Resources footprint include:

• 42.42 grams per tonne gold over 1.50 m in IM-26-001 at 111 m vertical depth (Figure 2), including:
  • 122.50 g/t Au over 0.50 m, and
  • 3.17 g/t Au over 0.50 m
• 6.40 g/t Au over 6.30 m in IM-26-001 at 326 m vertical depth, including:
  • 40.60 g/t Au over 0.50 m, and
  • 19.10 g/t Au over 0.70 m, and
  • 4.56 g/t Au over 0.70 m
• 6.17 g/t Au over 7.10 m in IM-26-003 at 517 m vertical depth, including:
  • 73.20 g/t Au over 0.50 m, and
  • 10.40 g/t Au over 0.50 m, and
  • 2.21 g/t Au over 0.50 m
• 13.19 g/t Au over 3.60 m in IM-26-003 at 596 m vertical depth, including:
  • 73.10 g/t Au over 0.50 m, and
  • 15.95 g/t Au over 0.50 m, and
  • 2.25 g/t Au over 0.60 m, and
  • 1.93 g/t Au over 0.50 m
• 4.87 g/t Au over 5.90 m in IM-26-003 at 524 m vertical depth, including:
  • 29.00 g/t Au over 0.50 m, and
  • 17.15 g/t Au over 0.50 m, and
  • 4.56 g/t Au over 0.50 m, and
  • 2.81 g/t Au over 0.50 m
• 6.61 g/t Au over 4.50 m in IM-26-003 at 425 m vertical depth (Figure 3), including:
  • 41.00 g/t Au over 0.50 m, and
  • 5.84 g/t Au over 0.50 m, and
  • 4.40 g/t Au over 0.50 m, and
  • 2.91 g/t Au over 0.50 m, and
  • 2.24 g/t Au over 0.50 m
• 2.79 g/t Au over 8.25 m in IM-26-001 at 436 m vertical depth, including:
  • 19.65 g/t Au over 0.50 m, and
  • 9.30 g/t Au over 0.50 m, and
  • 3.06 g/t Au over 0.50 m, and
  • 6.36 g/t Au over 0.50 m, and
  • 4.03 g/t Au over 0.50 m
• 13.83 g/t Au over 1.55 m in IM-26-003 at 277 m vertical depth, including:
  • 38.70 g/t Au over 0.55 m
• 8.81 g/t Au over 2.25 m in CM-26-001 at 370 m vertical depth, including:
  • 23.10 g/t Au over 0.50 m, and
  • 16.00 g/t Au over 0.50 m
• 17.32 g/t Au over 1.00 m in CM-25-003 at 371 m vertical depth, including:
  • 33.10 g/t Au over 0.50 m

Complete assay highlights, including true width estimates and vertical depths from surface, are presented in Tables 1 and 2, split by relative position within and outside of the current footprint of the Cariboo Gold Mineral Resources¹. Drill hole locations and collar orientations are listed in Table 2. Intervals not recovered by drilling were assigned zero grade. Top cuts have not been applied to high grade assays.

Figure 1: Plan view Cariboo deep and gap zone surface drilling select intercept highlights.

Figure 2: 42.42 g/t Au over 1.50 m in IM-26-001 at 111 m vertical depth.

Figure 3: 6.61 g/t au over 4.50 m in IM-26-003 at 425 m vertical depth.

Figure 4: 2.79 g/t Au over 8.25 m in IM-26-001 at 436 m vertical depth.

Figure 5: Long section displaying Cariboo deeps and gap zone surface drilling with select intercept highlights.

DISCUSSION OF RESULTS

• Results to date confirm the presence of high-grade mineralization at depth and reinforce the strong potential for resource growth upside well beyond the 350 m average of depth of the current deposit.
• The consistency of broad high-grade intercepts in drill holes testing through and under the Shaft-Mosquito Gap, as seen in IM-26-002 and IM-26-003 (Figure 5), suggests high potential for near mine upside within this previously undertested zone.
• Significant number of intercepts in Table 1 (length weighted assay composites and individual samples>=1.8 g/t Au within the footprint of the current Mineral Resources) demonstrate notable continuity and possible resource upside potential.
• All eleven holes released encountered assay composites >=1.8 g/t Au, with ten holes encountering assay composites >=1.8 g/t Au outside the current Mineral Resources. One hole (CM-25-001) was stopped early at 114 m due to historical workings intercepted at 108 m downhole.

ABOUT THE CARIBOO DEEP PROGRAM

This ongoing program is designed to test the potential for vein corridor extensions of the existing Cariboo Gold deposit down to vertical depths of up to 800 m. The defined limit of the existing Cariboo Gold Mineral Resources and Mineral Reserves reflects limitations imposed by local topography and the practical depth of surface drills for achieving intercept spacing required for a mineral resource estimate, which is approximately 350 m vertical on average across the 4.4 km strike of the deposit. The program is also designed to target under-tested gap areas between defined deposit zones where historic mining was observed, including, for example, the ~1.2 km gap between the Shaft and Mosquito Creek Zones. Drillholes collared within the defined resource provide additional infill intercepts at shallower depths. Directional drilling has been implemented on select drillholes for higher precision targeting to depth.

Ongoing surface drilling will continue to systematically test down-dip of the current resource to a maximum along strike target spacing of 300 m. Additional holes at target spacing 50 m near surface and 100 m at depth will be drilled to test continuity around the most prospective intercepts outside the current Mineral Resources footprint. New lithologic data incorporated into updated and extended geologic models is expected to aid in further refining of at depth targets.

Table 1: Length weighted assay composites and individual samples>=1.8 g/t for Cariboo Deep diamond drill intercepts within the footprint of the current resource.

Table 2: Length weighted assay composites and individual samples>=1.8 g/t for Cariboo Deep diamond drill intercepts outside the footprint of the current resource.

Table 3: Surface DD collar locations, orientations, and max drillhole depths. Negative dips point down.

CORPORATE NAME CHANGE

As previously announced, Osisko Development Corp. intends to change its name to Osisko Gold Group Inc., after receiving shareholder approval for the change of name at its annual and special meeting held on June 23, 2026. Also as previously announced, the Company intends to change the trading symbol for its common shares on both the TSX Venture Exchange and the New York Stock Exchange from “ODV” to “OGG”.

The Company anticipates the legal change in name to become effective on July 20, 2026, and expects its common shares to begin trading under the new symbol “OGG” on both the TSXV and the NYSE effective at market open on July 20, 2026. The trading symbols for the Company’s common share purchase warrants are also expected to change effective at market open on July 20, 2026: (i) warrants trading under “ODVWZ” on the Nasdaq Stock Market and “ODV.WT.U” on the TSXV will change to “OGGWZ” and “OGG.WT.U”, respectively; and (ii) warrants trading on the TSXV under “ODV.WT.A” and “ODV.WT.V” will change to “OGG.WT.A”, and “OGG.WT.V”, respectively.

In connection with the name and symbol changes of the Company’s listed securities, the CUSIPs and ISIN’s of the Company’s securities will change as well. For the Company’s common shares, the CUSIP and ISIN will be changed to 68827X105 and CA68827X1050, respectively. In addition, the following new CUSIP and ISIN numbers have been assigned to the warrants of the Company: (i) OGG.WT.A: CUSIP 68827X139 and ISIN CA68827X1399; (ii) OGG.WT.U (TSXV) and OGGWZ (Nasdaq): CUSIP 68827X113 and ISIN CA68827X1134; and (iii) OGG.WT.V: CUSIP 68827X121 and ISIN CA68827X1217.

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 area that extends approximately 77-kilometres from 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

Scientific and technical information relating to the Cariboo Gold Project and the 2025 feasibility study 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).

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 in its entirety, including all assumptions, parameters, qualifications, limitations and methods therein. The Technical Report is intended to be read as a whole, and sections should not be read or relied upon out of context. The Technical Report was prepared in accordance with National Instrument 43-101 – Standards of Disclosure for Mineral Projects 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, verified and approved by Scott Smith, P. Geo., Vice President, Exploration of Osisko Development, a “qualified person” within the meaning of NI 43-101. Verification includes core photo and three-dimensional review of logged drillhole data and assays consistent with the Company’s standard procedures.

Quality Assurance – Quality Control

HQ and NQ diameter drill core is cut (halved) on site at the Cariboo Project subsequent to QAQC checks for logging and sampling errors. BQ core (limited to directional intervals) are whole core sampled following daily QAQC. Any BQ intercepts external to the footprint of the current resource (none herein) are to be excluding from informing any new resource shapes. Quality control (QC) samples are inserted at regular intervals in the sample stream, including blanks and reference materials with all sample shipments to monitor laboratory performance. Samples are bagged, labeled, sealed with numbered security tags

All 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 50 gram 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 by fire-assay with gravimetric finish (upper limit 10,000 ppm). Select samples containing visible gold and/or cosalite are flagged during logging for an additional 1,000-gram screen metallic fire assay to ensure accurate quantification of any coarse fraction. All 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).

End Notes (excluding tables)

1. Refer to the Cariboo Technical Report for the assumptions, qualifications and limitations relating to disclosure about the Cariboo Gold Project and the 2025 feasibility study on the Cariboo Gold Project.

ABOUT OSISKO DEVELOPMENT CORP.

Osisko Development Corp. is a continental North American gold development company focused on past producing mining camps with district scale potential. The Company’s objective is to become an intermediate gold producer through the development of its flagship, fully permitted, 100%-owned Cariboo Gold Project, located in central British Columbia, Canada. Its project pipeline is complemented by the Tintic Project located in the historic East Tintic mining district in Utah, U.S.A., a brownfield property with significant exploration potential, extensive historical mining data, and access to established infrastructure. Osisko Development is focused on developing long-life mining assets in mining-friendly jurisdictions while maintaining a disciplined approach to capital allocation, development risk management, and mineral inventory growth.