| omniX labsOther | Other | New York City, United States | Vikram Sirkar | omnixlabs.com |
| OncoReviveOther | Other | Melbourne, Australia | — | oncorevive.com |
| One ModelB2B | B2B | San Antonio, United States | Steve Touw | onemodel.co |
| One PlanFintech | Fintech | Cantonments, Ghana | — | oneplan.finance |
| 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 |
| 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 |
| Onward DeliveryOther | Other | Denver, United States | — | onwarddelivery.com |
| Open BionicsOther | Other | Bristol, United Kingdom | Joel Gibbard | openbionics.com |
| Open to the offerOther | Other | Milford, United States | — | opentotheoffer.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 |
| 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 |
| 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 |
| OrdanaB2B | B2B | New York City, United States | — | blu.ai |
| OrdercubeOther | Other | Grafing b. München, Germany | — | ordercube.de |
| Orderly HealthB2B | B2B | Denver, United States | — | orderlyhealth.com |
| OrdermarkOther | Other | Los Angeles, United States | Alex Canter | ordermark.com |
| OrderNerdOther | Other | Peachtree Corners, United States | — | ordernerd.com |
| Origami Space DevelopmentB2B | B2B | Berthoud, United States | — | origamispace.com |
| OriginFintech | Fintech | London, United Kingdom | Max Boonen | originmarkets.com |
| OriientOther | Other | Tel Aviv, Israel | Ran Steinfeld | oriient.me |
| Orion FinanceB2B | B2B | Newark, United States | — | orionfinance.ai |
| OtrafyB2B | B2B | Saint Paul, United States | — | otrafy.com |
| OtterSpaceIndustrials | Industrials | Seattle, United States | — | inotterspace.com |
| OTTOConsumer | Consumer | Memphis, United States | — | myotto.net |
| Our Leg UpIndustrials | Industrials | Bundoora, Australia | — | ourlegup.com |
| OutreachOther | Other | Seattle, United States | Manny Medina | outreach.io |
| OverstoryOther | Other | Amsterdam, Netherlands | Ton Snelder | 20tree.ai |
| Overwatch AIOther | Other | Barcelona, Spain | — | overwatch-ai.com |
| Ovia HealthOther | Other | Boston, United States | Paris Wallace | oviahealth.com |
| OvidaB2B | B2B | Nottingham, United Kingdom | — | ovida.org |
| Owlet Baby CareHealthcare | Healthcare | Lehi, United States | Kurt Workman | owletcare.com |
| Own UpFintech | Fintech | Boston, United States | Patrick Boyaggi | ownup.com |
| Ozeki Legal TechnologiesOther | Other | Mill Valley, United States | — | ozeki.ai |
| P.S. Bridal RentalOther | Other | Hertfordshire, United Kingdom | — | psbridal.co.uk |
| PackEducation | Education | Firenze, Italy | — | thepack.tech |