| Oklahoma Center for NonprofitsEIN 73-1501645 | Community Development | OK | $1,690,215Small | 90.25 |
| Oklahoma Medical Research FoundationEIN 73-0580274 | Health | OK | $75,113,337Large | 93.94 |
| Old Dartmouth Historical Society/New Bedford Whaling MuseumEIN 04-2104805 | Arts, Culture, Humanities | MA | $4,527,585Mid-sized | 93.25 |
| Old Globe TheatreEIN 95-1543396 | Arts, Culture, Humanities | CA | $25,227,945Large | 90.59 |
| Old Newsboys' Goodfellow Fund of DetroitEIN 38-6061491 | Human Services | MI | $1,763,242Small | 91.22 |
| Ole Miss Athletics FoundationEIN 64-0474850 | Education | MS | $41,714,200Large | 93.73 |
| Oliver ScholarsEIN 13-3248876 | Education | NY | $4,018,563Mid-sized | 96.97 |
| Omaha SymphonyEIN 47-6039304 | Arts, Culture, Humanities | NE | $7,876,033Mid-sized | 95.83 |
| On the BoardsEIN 91-1081983 | Arts, Culture, Humanities | WA | $1,635,622Small | 90.81 |
| On the MoveEIN 75-3149095 | Education | CA | $4,571,332Mid-sized | 90.58 |
| One Acre FundEIN 20-3668110 | International | IL | $98,006,834Large | 90.19 |
| One Heart World-WideEIN 20-0443243 | International | CA | $2,298,326Small | 96.66 |
| One Israel Fund Ltd.EIN 11-3195338 | International | NY | $3,470,381Small | 92.50 |
| One World Children's FundEIN 77-0479205 | International | CA | $2,242,722Small | 98.23 |
| OneSightEIN 31-1385607 | International | OH | $19,899,787Large | 92.61 |
| OneSkyEIN 95-4714047 | International | CA | $6,328,760Mid-sized | 92.63 |
| Open Arms of MinnesotaEIN 41-1681317 | Human Services | MN | $3,381,410Small | 91.45 |
| Open Door Community HouseEIN 58-0601980 | Human Services | GA | $1,511,733Small | 92.44 |
| Open Doors KalamazooEIN 23-7088427 | Human Services | MI | $1,150,677Small | 94.14 |
| Open SchoolEIN 93-0757378 | Education | OR | $5,210,734Mid-sized | 91.78 |
| OPERA AmericaEIN 20-3520577 | Arts, Culture, Humanities | NY | $7,895,530Mid-sized | 91.83 |
| Opera OmahaEIN 47-6032795 | Arts, Culture, Humanities | NE | $3,861,688Mid-sized | 96.90 |
| Opera San JoséEIN 77-0009773 | Arts, Culture, Humanities | CA | $4,434,463Mid-sized | 92.72 |
| Opera Theatre of Saint LouisEIN 43-0821958 | Arts, Culture, Humanities | MO | $10,222,016Mid-sized | 93.44 |
| Operation Bootstrap AfricaEIN 41-6051952 | International | MN | $838,344Small | 95.15 |
| Operation BreakthroughEIN 43-0971560 | Human Services | MO | $8,616,112Mid-sized | 95.72 |
| Operation First ResponseEIN 20-1622436 | Human Services | VA | $803,443Small | 91.16 |
| Operation Food SearchEIN 43-1241854 | Human Services | MO | $33,636,061Large | 94.03 |
| Operation FuelEIN 06-1253091 | Human Services | CT | $4,590,024Mid-sized | 92.66 |
| Operation GratitudeEIN 20-0103575 | Human Services | CA | $10,112,056Mid-sized | 92.38 |
| Operation KindnessEIN 75-1553350 | Animals | TX | $4,625,577Mid-sized | 90.01 |
| 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 Fund Community Development EIN 31-1719434 | Community Development | CA | $19,078,312Large | 91.49 |
| 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 Community FoundationEIN 33-0378778 | Community Development | CA | $64,365,979Large | 94.37 |
| Orange County Rescue MissionEIN 95-2479552 | Human Services | CA | $16,241,092Large | 91.21 |
| Orangewood FoundationEIN 95-3616628 | Human Services | CA | $9,272,668Mid-sized | 92.50 |
| Orangutan Foundation InternationalEIN 95-4112467 | Animals | CA | $1,603,050Small | 93.42 |
| Orbis InternationalEIN 23-7297651 | International | NY | $209,113,219Large | 95.47 |
| Ordway Center for the Performing ArtsEIN 41-1428998 | Arts, Culture, Humanities | MN | $21,131,100Large | 91.33 |
| Oregon Ballet TheatreEIN 93-1009305 | Arts, Culture, Humanities | OR | $7,234,406Mid-sized | 90.58 |
| Oregon Food BankEIN 93-0785786 | Human Services | OR | $67,194,439Large | 90.96 |
| Oregon Health & Science University FoundationEIN 23-7083114 | Education | OR | $130,218,507Large | 91.70 |
| Oregon Historical SocietyEIN 93-0391599 | Arts, Culture, Humanities | OR | $8,175,545Mid-sized | 90.76 |
| Oregon Humane SocietyEIN 93-0386880 | Animals | OR | $14,537,262Large | 92.77 |
| Oregon Lions Sight & Hearing FoundationEIN 93-6041506 | Human Services | OR | $1,494,976Small | 96.45 |
| Oregon Museum of Science and IndustryEIN 93-0402877 | Arts, Culture, Humanities | OR | $21,468,218Large | 92.80 |
| Oregon Natural Desert AssociationEIN 94-3098621 | Environment | OR | $1,631,248Small | 94.83 |
| Oregon Public BroadcastingEIN 93-0814638 | Arts, Culture, Humanities | OR | $31,776,224Large | 90.24 |
| Oregon Zoo FoundationEIN 93-0718337 | Animals | OR | $7,970,518Mid-sized | 94.75 |
| Organization for Autism ResearchEIN 54-2062167 | Health | VA | $1,326,919Small | 95.05 |
| Orlando Shakespeare TheaterEIN 59-2931698 | Arts, Culture, Humanities | FL | $3,747,155Mid-sized | 91.91 |
| Orphan Grain TrainEIN 31-1614650 | International | NE | $49,465,762Large | 97.17 |
| ORPHANetworkEIN 54-1983817 | International | VA | $5,817,562Mid-sized | 95.47 |
| ORT AmericaEIN 13-5562424 | Education | NY | $15,138,099Large | 90.27 |
| Oshkosh Area United WayEIN 39-1017908 | Community Development | WI | $1,374,471Small | 95.31 |