| 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 Shakespeare FestivalEIN 93-0407022 | Arts, Culture, Humanities | OR | $41,256,313Large | 87.08 |
| Oregon SymphonyEIN 93-0446527 | Arts, Culture, Humanities | OR | $18,547,539Large | 89.51 |
| Oregon Zoo FoundationEIN 93-0718337 | Animals | OR | $7,970,518Mid-sized | 94.75 |
| 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 |
| Organization for Tropical StudiesEIN 56-2125831 | Education | NC | $8,180,616Mid-sized | 83.68 |
| Orlando Museum of ArtEIN 59-0910352 | Arts, Culture, Humanities | FL | $3,686,650Mid-sized | 85.17 |
| Orlando Philharmonic OrchestraEIN 59-3058884 | Arts, Culture, Humanities | FL | $5,112,662Mid-sized | 89.11 |
| Orlando Science CenterEIN 59-0896343 | Arts, Culture, Humanities | FL | $9,959,285Mid-sized | 89.48 |
| Orlando Shakespeare TheaterEIN 59-2931698 | Arts, Culture, Humanities | FL | $3,747,155Mid-sized | 91.91 |
| Orlando Union Rescue MissionEIN 59-1035082 | Human Services | FL | $5,899,987Mid-sized | 84.92 |
| Orphan Grain TrainEIN 31-1614650 | International | NE | $49,465,762Large | 97.17 |
| Orphan HelpersEIN 54-1995429 | Religion | VA | $605,140Small | 80.16 |
| ORPHANetworkEIN 54-1983817 | International | VA | $5,817,562Mid-sized | 95.47 |
| Orpheus Chamber OrchestraEIN 23-7362572 | Arts, Culture, Humanities | NY | $2,993,130Small | 78.95 |
| ORT AmericaEIN 13-5562424 | Education | NY | $15,138,099Large | 90.27 |
| Orthopaedic Research and Education FoundationEIN 36-6009467 | Health | IL | $5,165,708Mid-sized | 69.21 |
| Oshkosh Area Community FoundationEIN 39-2034571 | Community Development | WI | $7,573,460Mid-sized | 86.28 |
| Oshkosh Area Humane SocietyEIN 39-1709813 | Animals | WI | $1,116,589Small | 86.10 |
| Oshkosh Area United WayEIN 39-1017908 | Community Development | WI | $1,374,471Small | 95.31 |
| Osteogenesis Imperfecta FoundationEIN 23-7076021 | Health | MD | $1,948,774Small | 93.31 |
| Ounce of Prevention FundEIN 36-3186328 | Human Services | IL | $56,161,326Large | 90.76 |
| OUR CenterEIN 74-2448346 | Human Services | CO | $4,444,201Mid-sized | 93.05 |
| Our Companions Animal RescueEIN 41-2047734 | Animals | CT | $1,049,781Small | 95.20 |
| Our Daily Bread MinistriesEIN 38-1613981 | Religion | MI | $46,676,641Large | 89.50 |
| OUR HOUSE Grief Support CenterEIN 33-0529915 | Human Services | CA | $2,122,838Small | 90.85 |
| Our House, ArkansasEIN 71-0653846 | Human Services | AR | $3,233,707Small | 100.00 |
| Our House, GeorgiaEIN 58-1743333 | Human Services | GA | $3,302,626Small | 92.21 |
| Our KidsEIN 58-1830327 | Human Services | TN | $1,502,257Small | 85.53 |
| Our Lady's InnEIN 43-1213751 | Human Services | MO | $2,584,254Small | 91.69 |
| Our Little HavenEIN 43-1567500 | Human Services | MO | $2,953,702Small | 95.16 |
| Our Military KidsEIN 56-2483648 | Human Services | VA | $2,091,157Small | 93.65 |
| Our Piece of the PieEIN 06-0939659 | Human Services | CT | $8,879,851Mid-sized | 88.22 |
| Out & Equal Workplace AdvocatesEIN 02-0681855 | Human and Civil Rights | CA | $5,530,425Mid-sized | 89.54 |
| Outdoor Discovery Center Macatawa GreenwayEIN 38-2461102 | Environment | MI | $2,992,451Small | 88.77 |
| Outdoors for All FoundationEIN 91-1085999 | Human Services | WA | $1,795,306Small | 85.25 |
| Outreach InternationalEIN 43-1164177 | International | MO | $3,571,376Mid-sized | 98.00 |
| Outreach360EIN 58-2197227 | International | AZ | $1,334,211Small | 89.84 |
| OutRight Action InternationalEIN 94-3139952 | Human and Civil Rights | NY | $2,691,363Small | 88.00 |
| OutServe-SLDNEIN 52-1845000 | Human and Civil Rights | DC | $489,839Small | 86.77 |
| Ovarian Cancer Research AllianceEIN 13-3806788 | Health | NY | $10,805,177Mid-sized | 93.28 |
| Overlook FoundationEIN 51-0194054 | Health | NJ | $6,419,341Mid-sized | 84.96 |
| Overtown Youth CenterEIN 65-1048896 | Education | FL | $3,579,226Mid-sized | 88.81 |
| Oxfam AmericaEIN 23-7069110 | International | MA | $91,570,741Large | 83.65 |
| Ozarks Food HarvestEIN 43-1426384 | Human Services | MO | $30,424,252Large | 97.46 |