Grapevines
- Oakland Community Alliance
- Oakland Stock
- Oasis House for Healing and Restoration (Domestic Violence organization)
- Occidental College Alumni Giving Circle
- OFJCC Chai Giving Circle
- Ohio Women for Blue States
- Ojai Women’s Fund
- OJCF Giving Council
- Oklahoma City Business Leaders for Good
- Oklahoma Native Assets Coalition: Serving Native Communities Across the U.S.
- Olim Making Change
- Olive Branch Giving Circles
- Omaha Venture Group
- Omar's Giving Circle
- Omicron Sigma Zeta Foundation
- One Earth Alliance
- One Percent for Moms
- One Sky Giving Circle
- One State at a Time
- Open Hearts, Open Purses
- Orange County Latino Giving Circle
- Orange County Women’s Giving Circle
- Orchid Giving Circle
- Oregon Jewish Community Foundation Giving Council
- Ottawakent Women Who Care
- Ouray County Giving Girls
- Our Sarasota Fund
- Our soldier’s
- Our Strategic Giving Circle: Leveraging Power in State Legislatures
- OUT Fund
- Outgiving Circle
- Overton Oversubscribe Giving Circle
- Oxfam-Asian Circle
- Oxygen