| Ohio Environmental CouncilEIN 31-0805578 | Environment | OH | $2,085,734Small | 83.94 |
| Ohio United WayEIN 31-0619209 | Community Development | OH | $872,862Small | 83.50 |
| Ojai Music FestivalEIN 95-2122508 | Arts, Culture, Humanities | CA | $1,980,707Small | 79.08 |
| Okizu FoundationEIN 68-0291178 | Health | CA | $2,017,839Small | 86.23 |
| Oklahoma Arts InstituteEIN 73-1021165 | Education | OK | $2,394,342Small | 86.44 |
| Oklahoma Center for NonprofitsEIN 73-1501645 | Community Development | OK | $1,690,215Small | 90.25 |
| Oklahoma Contemporary Arts CenterEIN 73-1334271 | Arts, Culture, Humanities | OK | $2,546,424Small | 77.84 |
| Oklahoma Council of Public AffairsEIN 73-1436375 | Research and Public Policy | OK | $2,173,852Small | 75.57 |
| Oklahoma Humane SocietyEIN 20-8446621 | Animals | OK | $3,137,627Small | 89.38 |
| Oklahoma Institute for Child AdvocacyEIN 73-1192768 | Human Services | OK | $894,750Small | 80.87 |
| Olbrich Botanical SocietyEIN 39-1357247 | Environment | WI | $2,874,467Small | 88.26 |
| Old Newsboys' Goodfellow Fund of DetroitEIN 38-6061491 | Human Services | MI | $1,763,242Small | 91.22 |
| Old Savannah City MissionEIN 31-1810829 | Human Services | GA | $1,200,200Small | 71.74 |
| Oldways Preservation TrustEIN 04-3031978 | Research and Public Policy | MA | $2,135,083Small | 85.00 |
| Oliver Gospel MissionEIN 57-6027750 | Human Services | SC | $3,388,839Small | 88.44 |
| Olympia Union Gospel MissionEIN 91-1680748 | Human Services | WA | $1,823,684Small | 70.48 |
| On PointEIN 68-0533402 | Human Services | TN | $1,327,471Small | 89.10 |
| On the BoardsEIN 91-1081983 | Arts, Culture, Humanities | WA | $1,635,622Small | 90.81 |
| On The RiseEIN 04-3290689 | Human Services | MA | $1,227,796Small | 89.22 |
| Oncology Nursing Society FoundationEIN 25-1410081 | Health | PA | $1,399,783Small | 88.19 |
| One FamilyEIN 54-2076936 | Human Services | MA | $1,493,655Small | 88.22 |
| One Heart World-WideEIN 20-0443243 | International | CA | $2,298,326Small | 96.66 |
| One HeartlandEIN 39-1763115 | Health | MN | $1,622,396Small | 85.75 |
| One Israel Fund Ltd.EIN 11-3195338 | International | NY | $3,470,381Small | 92.50 |
| One Million DegreesEIN 42-1710230 | Education | IL | $2,667,285Small | 87.16 |
| One VoiceEIN 95-3900124 | Human Services | CA | $1,716,351Small | 87.68 |
| One Way FarmEIN 31-0926233 | Human Services | OH | $1,308,959Small | 78.86 |
| One World Children's FundEIN 77-0479205 | International | CA | $2,242,722Small | 98.23 |
| OneFamilyEIN 11-3585917 | Human Services | NJ | $1,086,033Small | 82.43 |
| Open Air Campaigners-USAEIN 36-2420024 | Religion | PA | $1,402,536Small | 78.77 |
| Open Arms of MinnesotaEIN 41-1681317 | Human Services | MN | $3,381,410Small | 91.45 |
| Open BooksEIN 20-4830666 | Education | IL | $2,045,810Small | 84.17 |
| Open Door Community HouseEIN 58-0601980 | Human Services | GA | $1,511,733Small | 92.44 |
| Open Door MinistriesEIN 56-1576543 | Human Services | NC | $1,696,558Small | 74.89 |
| Open Door Mission, HoustonEIN 76-0146890 | Human Services | TX | $2,973,113Small | 80.91 |
| Open Doors KalamazooEIN 23-7088427 | Human Services | MI | $1,150,677Small | 94.14 |
| Open Your Heart to the Hungry and HomelessEIN 36-3488089 | Human Services | MN | $784,716Small | 89.50 |
| OpenhouseEIN 94-3337955 | Human Services | CA | $1,606,068Small | 89.17 |
| Opera MemphisEIN 62-0785544 | Arts, Culture, Humanities | TN | $1,623,169Small | 83.52 |
| Operation AccessEIN 94-3180356 | Health | CA | $1,466,203Small | 86.33 |
| Operation Bootstrap AfricaEIN 41-6051952 | International | MN | $838,344Small | 95.15 |
| Operation First ResponseEIN 20-1622436 | Human Services | VA | $803,443Small | 91.16 |
| Operation NightwatchEIN 91-0964027 | Human Services | WA | $1,154,918Small | 87.82 |
| Operation Second ChanceEIN 20-2624345 | Human Services | MD | $1,112,969Small | 91.85 |
| Operation ShoestringEIN 64-0471554 | Human Services | MS | $1,215,347Small | 93.27 |
| Operation USAEIN 95-3504080 | International | CA | $3,338,684Small | 92.92 |
| Opportunity, Inc. of Palm Beach CountyEIN 59-0624429 | Education | FL | $1,751,901Small | 94.06 |
| Optimist International FoundationEIN 23-7102928 | Education | MO | $1,513,808Small | 90.41 |
| Orange County CoastkeeperEIN 33-0847892 | Environment | CA | $1,772,988Small | 94.74 |
| Orange County Museum of ArtEIN 95-1660847 | Arts, Culture, Humanities | CA | $2,605,178Small | 85.08 |
| Orangutan Foundation InternationalEIN 95-4112467 | Animals | CA | $1,603,050Small | 93.42 |
| Oregon Energy FundEIN 93-1029893 | Human Services | OR | $1,245,206Small | 89.18 |
| Oregon Lions Sight & Hearing FoundationEIN 93-6041506 | Human Services | OR | $1,494,976Small | 96.45 |
| Oregon Natural Desert AssociationEIN 94-3098621 | Environment | OR | $1,631,248Small | 94.83 |
| Organ Donation And Transplant Association Of AmericaEIN 30-0339531 | Health | FL | $117,858Small | 26.93 |
| Organic Consumers AssociationEIN 41-1908341 | Environment | MN | $3,495,920Small | 87.44 |
| Organic Farming Research FoundationEIN 77-0252545 | Research and Public Policy | CA | $871,992Small | 87.24 |
| Organization for Autism ResearchEIN 54-2062167 | Health | VA | $1,326,919Small | 95.05 |
| Orphan HelpersEIN 54-1995429 | Religion | VA | $605,140Small | 80.16 |
| Orpheus Chamber OrchestraEIN 23-7362572 | Arts, Culture, Humanities | NY | $2,993,130Small | 78.95 |