| | Computer Software | San Francisco | CA | $50 - 100M |
| Office DepotOffice Depot & OfficeMax: Office Supplies and Furniture | Business Supplies and Equipment | Boca Raton | FL | > $1B |
| OYO RoomsHotel Booking India, Branded hotels, Affordable Stays ??? OYO Rooms | Hospitality | Gurgaon | HR | $1 - 10M |
| OoyalaOoyala | Deliver Content that Connects | Marketing and Advertising | Santa Clara | CA | $50 - 100M |
| optionsXpressOptions Trading, Stock Trading & Futures Trading at optionsXpress | Financial Services | Chicago | IL | $250 - 500M |
| | Computer Software | New York | NY | $10 - 50M |
| | Medical Practice | San Francisco | CA | $10 - 50M |
| | Telecommunications | London | | $250 - 500M |
| | E-Learning | Sydney | NSW | $100 - 250M |
| ownCloudownCloud | Secure File Access and File Sharing | Information Technology and Services | Lexington | MA | $10 - 50M |
| | | | | |
| onPeak | GESonPeak - the proactive event housing partner of savvy planners | Events Services | Chicago | IL | $1 - 10M |
| | Higher Education | Melbourne | Victoria | $50 - 100M |
| OnlinePizzaBest??ll mat hemk??rning online | OnlinePizza.se | Information Technology and Services | Stockholm | Kungsholmen | $1 - 10M |
| | Internet | New York | NY | $1 - 10M |
| | Internet | Seattle | WA | $1 - 10M |
| OnDeckSmall Business Loans and Business Credit Insights from On Deck Capital | Financial Services | New York | NY | $50 - 100M |
| | Hospitality | London | England | $10 - 50M |
| OmnitracsFleet Management Systems & Software | Omnitracs | Transportation/Trucking/Railroad | San Diego | CA | $1 - 10M |
| | Apparel & Fashion | Berlin | Berlin | $10 - 50M |
| | Financial Services | Omaha | NE | $1 - 10M |
| ONVIA, Inc.Leading Commerce Intelligence in B2G | Onvia | Information Technology and Services | Seattle | WA | $10 - 50M |
| | Marketing and Advertising | New York | NY | $1 - 10M |
| OPSWATVulnerability Scanning | Content Disarm and Reconstruction: OPSWAT | Computer Software | San Francisco | CA | $10 - 50M |
| | Consumer Goods | Nashville | TN | $500M - 1B |
| | Banking | Aurora | IL | $100 - 250M |
| | Internet | Oklahoma City | OK | $10 - 50M |
| OfficiteProfessional Medical Websites, Top Medical Practice Website Firm | Officite | Internet | Downers Grove | IL | $1 - 10M |
| | Computer Hardware | | | |
| OpternativeVision Tests & Prescriptions From Home | Opternative | Hospital & Health Care | Chicago | IL | $1 - 10M |
| | Computer Software | New York | NY | $10 - 50M |
| | Information Technology and Services | Burlington | MA | $1 - 10M |
| OptionsANIMALOptionsANIMAL – The Only Complete Options Trading Education | Investment Management | Lehi | UT | $1 - 10M |
| OrlandoJobs.comOrlando Jobs | Search for Orlando Florida Jobs, Accounting, Marketing, Sales, Call Center | Staffing and Recruiting | Orlando | FL | $1 - 10M |
| Online RewardsEmployee Recognition, Channel Incentives, Customer Loyalty | Online Rewards | Information Technology and Services | Dallas | TX | $10 - 50M |
| ObserveITObserveIT | Identify, Investigate & Eliminate Insider Threat | Computer Software | NY | NY | $10 - 50M |
| Oxygen8 GroupMulti-Channel Voice, Payment and SMS Solutions - Oxygen8 | Telecommunications | | | $10 - 50M |
| | Biotechnology | Oxford | | $10 - 50M |
| | Business Supplies and Equipment | Auckland | | $100 - 250M |
| | Furniture | Northbrook | IL | $0 - 1M |
| | Internet | Rancho Dominguez | CA | $10 - 50M |
| | Law Practice | Toronto | ON | $500M - 1B |
| | Nonprofit Organization Management | Brattleboro | VT | $1 - 10M |
| OneDesk Inc.Software for product management, project management, & help desk | Computer Software | Montr | QC | $1 - 10M |
| OverOpsDetect and fix production errors | OverOps | Computer Software | San Francisco | California | |
| | Information Technology and Services | | | $1 - 10M |
| | Computer Software | North Sydney | NSW | $10 - 50M |
| | Information Technology and Services | Copenhagen | | $1 - 10M |
| | Market Research | London | | $1 - 10M |
| | Law Practice | READING | Oxfordshire | |
| | Think Tanks | | | $1 - 10M |
| OneSpotContent Marketing Software | OneSpot | Marketing and Advertising | | | $1 - 10M |
| | Computer Software | Los Angeles | CA | $0 - 1M |
| O.Berk CompanyGlass, Plastic, Metal Bottles and Caps Wholesale - O.Berk | Packaging and Containers | Union | NJ | $1 - 10M |
| | Nonprofit Organization Management | LAS VEGAS | NV | $1 - 10M |
| | Construction | | | $500M - 1B |
| OwnBackupOwnBackup | Cloud to Cloud Backup and Recovery for the Enterprise | Internet | Fort Lee | NJ | $0 - 1M |
| | Internet | San Francisco | CA | $0 - 1M |
| | Information Technology and Services | Warsaw | Poland | $50 - 100M |
| On Device ResearchMobile market research - sampling, panel & survey technology | On Device Research | Market Research | London | | $1 - 10M |
| Opal LabsA Collaboration Platform for Brand Marketers - Opal | Computer Software | Portland | OR | $1 - 10M |
| | Nonprofit Organization Management | OLYMPIA | WA | $0 - 1M |
| | Internet | San Francisco | CA | $0 - 1M |
| ObilityB2B Demand Generation Services & Inbound Marketing Agency | Obility | Marketing and Advertising | Portland | OR | $1 - 10M |
| | Chemicals | Houston | TX | $50 - 100M |
| o9 Solutions, Inc.Cloud Business Solutions | Supply Chain Planning, Trade Promotion | o9 Solutions | Computer Software | Farmers Branch | TX | $0 - 1M |
| | Computer Software | Parker | CO | $1 - 10M |
| | Financial Services | San Jose | CA | $1 - 10M |
| | Financial Services | Sydney | NSW | |
| | Construction | Mumbai | Maharashtra | $50 - 100M |
| | Marketing and Advertising | Miami | FL | $1 - 10M |
| | Leisure, Travel & Tourism | Texarkana | AR | $0 - 1M |
| | Computer Software | Stockholm | . | $50 - 100M |
| | Computer Software | | | $1 - 10M |
| | Hospital & Health Care | San Francisco | CA | $50 - 100M |
| | Medical Devices | Holt | MI | $10 - 50M |
| ORMDigital Strategy, Digital Brand & Creative, Build & CMS | Marketing and Advertising | London | | $1 - 10M |
| OneKreateOneKreate | Create Once Share Everywhere | Media Production | Hollywood | FL | $50 - 100M |
| | Investment Management | Boston | MA | $500M - 1B |
| | Computer Software | Waltham | MA | $1 - 10M |
| Own the RoomOwn The Room? | Unleash Your Potential | Communication Training | Professional Training & Coaching | Verona | NJ | $1 - 10M |
| | Nonprofit Organization Management | Los Angeles | CA | $0 - 1M |
| OpenSky CorpOpenSky Corp | IT infrastructure, Security and Compliance Consulting | Information Technology and Services | Tolland | CT | $10 - 50M |
| | Telecommunications | Dublin 2 | Dublin | $10 - 50M |
| | Marketing and Advertising | So Paulo | SP | $1 - 10M |
| | Nonprofit Organization Management | Des Moines | IA | $0 - 1M |
| | Investment Management | Chicago | IL | $1 - 10M |
| OLRHome | OLR Quality. Delivered. | Information Technology and Services | Minneapolis | MN | $10 - 50M |
| | Information Technology and Services | Pune | Maharashtra | $10 - 50M |
| | Computer Software | West Palm Beach | FL | $10 - 50M |
| | Staffing and Recruiting | Reston | VA | $1 - 10M |
| | Information Technology and Services | Minneapolis | MN | $1 - 10M |
| OilgearOILGEAR: Best Under Pressure | Mechanical or Industrial Engineering | Milwaukee | WI | $100 - 250M |
| | Publishing | Oldenzaal | | $1 - 10M |
| | Financial Services | New York | NY | $10 - 50M |
| | Electrical/Electronic Manufacturing | Osborne Park | WA | $0 - 1M |
| Ocuco Limited| Ocuco | Software For Optical Retail and Optical Labs | Computer Software | Manchester | CT | $0 - 1M |
| OneE GroupTax Planning, Advice and Disputes | One E Group | Accounting | Bolton | England | |
| | Information Technology and Services | Suwanee | GA | $10 - 50M |
| | Information Technology and Services | Quebec | QC | $1 - 10M |