| Link | Oak Hill Advisors, LLC | New York | New York | | 1 | $15,020,000 | Content & Media Information Services |
| Link | Oak Ventures | Palo Alto | | Icrossing, Linux Network, Inc., Linux Networx, NextPage | 3 | $93,250,000 | |
| Link | Odyssey Venture Partners | Newport Beach | | FatPipe Networks | 1 | $750,000 | Communications Energy |
| Link | Ogden Angel Investors | | | Revinetix | 1 | | Software |
| Link | Ohio Biotech Group | | | Axial Biotech, Inc. | 1 | | Healthcare Services |
| Link | Olympus Angels | | | Revinetix | 1 | | Software |
| Link | Omninet Capital, LLC | Beverly Hills | California | | 1 | $3,810,000 | Energy |
| Link | OneLiberty Ventures | Cambridge | Massachusetts | Anesta | | | |
| Link | Ontario Teachers Pension Plan | Toronto | | | 1 | $14,350,000 | |
| Link | OpenView Venture Partners | Boston | Massachusetts | Attask, Inc | 1 | $7,000,000 | Content & Media Electronics Software |
| Link | Oracle Venture Fund | Redwood Shores | California | | 1 | $11,170,000 | Information Services Software Services |
| Link | Origin Partners | Boston | | Venafi, Inc | 1 | $2,470,000 | Electronics Software Security |
| Link | Outlook Ventures | San Francisco | | Echopass | 1 | | Content & Media Software Services Security |
| Link | Oxford BioScience Partners | Boston | | Coherex Medical, EmerGen | 3 | $11,700,000 | Biotechnology Electronics Energy Medical Devices |
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