| Olive TechnologiesB2B | B2B | Vancouver, Canada | Sean Lane | olive.app |
| Olli HealthB2B | B2B | New York City, United States | — | ollihomehealth.ai |
| OmnaraB2B | B2B | San Francisco, United States | | omnara.com |
| OmneeConsumer | Consumer | Saskatoon, Canada | — | omnee.ca |
| OmnioFintech | Fintech | London, United Kingdom | — | omnio.global |
| OmniPreSense CorporationOther | Other | San Jose, United States | Glenn Bottom | omnipresense.com |
| OmniscientB2B | B2B | — | — | omniscient-ai.io |
| omniX labsOther | Other | New York City, United States | Vikram Sirkar | omnixlabs.com |
| OncoReviveOther | Other | Melbourne, Australia | — | oncorevive.com |
| OnDeck AIB2B | B2B | Vancouver, Canada | | ondeckai.com |
| One ModelB2B | B2B | San Antonio, United States | Steve Touw | onemodel.co |
| One PlanFintech | Fintech | Cantonments, Ghana | — | oneplan.finance |
| One RobotIndustrials | Industrials | — | | onerobot.io |
| OneInbox.aiOther | Other | Beaverton, United States | — | oneinbox.ai |
| OnePipeFintech | Fintech | Lagos, Nigeria | Ope Adeoye | onepipe.io |
| OneScreen AIB2B | B2B | Boston, United States | Alok Ahuja | onescreen.ai |
| OneShotB2B | B2B | — | — | oneshot.ai |
| OneVillageIndustrials | Industrials | Washington, United States | — | onevillage.io |
| OnFrontiersOther | Other | New York City, United States | Matt Fates | onfrontiers.com |
| OnsiteOther | Other | Surry Hills, Australia | — | onsite.fm |
| OnStationB2B | B2B | Cleveland, United States | — | onstationapp.com |
| OnSwipeConsumer | Consumer | New York City, United States | Jason Baptiste | onswipe.com |
| OntoraB2B | B2B | San Francisco, United States | | ontora.com |
| Onward DeliveryOther | Other | Denver, United States | — | onwarddelivery.com |
| Opalite HealthHealthcare | Healthcare | — | | opalitehealth.com |
| Open BionicsOther | Other | Bristol, United Kingdom | Joel Gibbard | openbionics.com |
| Open to the offerOther | Other | Milford, United States | — | opentotheoffer.com |
| OpenAIB2B | B2B | — | Sam Altman | openai.com |
| OpenDrawing.aiB2B | B2B | Northglenn, United States | — | opendrawing.ai |
| OpenFarmingIndustrials | Industrials | Dubayy, United Arab Emirates | — | openfarming.earth |
| OpenFrameConsumer | Consumer | Cambridge, United States | — | getopenframe.com |
| OpenLoopB2B | B2B | Iowa City, United States | Caroline Pearson | openloophealth.com |
| OpenmartB2B | B2B | United States | — | openmart.ai |
| OpennoteConsumer | Consumer | San Francisco, United States | | opennote.com |
| OpenrollB2B | B2B | Stockholm, Sweden | | openroll.com |
| OpenSpecB2B | B2B | — | | openspec.dev |
| OpenWorkB2B | B2B | San Francisco, United States | | openworklabs.com |
| Oper CreditsFintech | Fintech | Antwerpen, Belgium | Wouter Blommaert | opercredits.com |
| OptiFab TechnologiesOther | Other | Kitchener, Canada | — | opti-fab.com |
| Optio.AiB2B | B2B | Tbilisi, Georgia | — | optio.ai |
| Options MDOther | Other | Los Angeles, United States | — | optionsmd.com |
| Optivolt LabsOther | Other | Burlingame, United States | — | optivoltlabs.com |
| Opusense AIOther | Other | — | | opusense.com |
| OradianFintech | Fintech | — | Ivan Mrvoš | oradian.com |
| Orange SliceB2B | B2B | — | | orangeslice.ai |
| ORBESOther | Other | San Jose, United States | — | orbes.space |
| OrbiseedB2B | B2B | Toronto, Canada | — | orbiseed.com |
| Orbit FabB2B | B2B | Santa Clara, United States | Daniel Faber | orbitfab.com |
| OrbitalB2B | B2B | — | — | orbital.tech |
| OrchidB2B | B2B | San Francisco, United States | | orchid.ai |