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| Oak Lawn Public Library (1) | Oak Park Public Library (3) |
| Oakland County Library/Adams-Pratt Law Library (1) | Oakland Museum/Museum of California Foundation (28) |
| Oakland Park Library (1) | Oakland Public Library (5) |
| Oakton Community College (3) | OASIS Institute (4) |
| Oberlin College (19) | Oberlin Heritage Center (3) |
| Oberlin Shansi Memorial Association (1) | Obion County Public Library (1) |
| Oboade Institute of African Culture (1) | Occidental College (8) |
| Ocean County Library (5) | Ocean County Library-Brick Branch (1) |
| Ocean Park Association (1) | Oceanside Public Library (1) |
| Ocmulgee Regional Library (1) | Oconee County Public Library (1) |
| Octagon Museum (1) | Oculus Films, Inc. (1) |
| Odell Public Library (1) | Ofeq Institute, Inc. (1) |
| Office of Public Libraries and Interlibrary Corporation (1) | Office of the Secretary of State, Illinois (1) |
| Ogdensburg Public Library (1) | Oglala Lakota College (3) |
| Oglebay Institute (1) | Oglethorpe University (2) |
| Ohio Association of Historical Societies and Museums (1) | Ohio Dominican College (4) |
| Ohio Educational Television Network Committee (1) | Ohio Genealogical Society (1) |
| Ohio Historical Foundation (1) | Ohio Historical Society (15) |
| Ohio Humanities Council (30) | Ohio State University Press (2) |
| Ohio State University Research Foundation (75) | Ohio State University, Main Campus (11) |
| Ohio University Telecommunications Center (1) | Ohio University, Athens (13) |
| Ohio Valley College (1) | Ohio Wesleyan University (4) |
| Ohoopee Regional Library (1) | Ohr-O'Keefe Museum of Art (2) |
| Oil, Chemical and Atomic Workers International Union (1) | Oklahoma Baptist University, Mabee Learning Center (1) |
| Oklahoma City University (2) | Oklahoma Historical Society (11) |
| Oklahoma Humanities Council (27) | Oklahoma Library Association (4) |
| Oklahoma Museums Association (1) | Oklahoma Panhandle State University (1) |
| Oklahoma State University, Stillwater (20) | Olathe District Schools (1) |
| Old Allentown Preservation Association, Inc. (1) | Old Colony YMCA/YWCA (1) |
| Old Dartmouth Historical Society (8) | Old Dominion University (6) |
| Old Dominion University Research Foundation (5) | Old Gaol Museum (1) |
| Old Independence Regional Museum (2) | Old Jail Art Center (1) |
| Old Jail Foundation (1) | Old Lyme-Phoebe Griffin Noyes Library Association (1) |
| Old Mines Area Historical Society (1) | Old North Foundation of Boston, Inc. (1) |
| Old Red Museum of Dallas County History & Culture (2) | Old Salem, Inc. (11) |
| Old Schwamb Mill (1) | Old South Association in Boston (3) |
| Old State House (1) | Old Stone House Museum (1) |
| Old Stone House of Brooklyn (1) | Old Sturbridge Village (18) |
| Old Swedes Church Foundation (1) | Old Town History Project (1) |
| Old Town Restoration, Inc. (1) | Old York Historical Society (3) |
| Oliver Wolcott Library (1) | Olmsted County Historical Society (3) |
| Olney Carnegie Public Library (2) | Omaha Public Library (2) |
| Omaho Public Library - W. Dale Clark Library (1) | Omega School of Excellence (1) |
| Omohundro Institute of Early American History and Culture (28) | ONE Institute/International Gay and Lesbian Archives (1) |
| Oneida Community Mansion House (2) | Oneida Historical Society (1) |
| Oneida Nation Museum (2) | Online Computer Library Center, Inc. (1) |
| Onondaga Community College (1) | Onondaga County Department of Parks and Recreation (3) |
| Onondaga County Public Library (2) | Onondaga Free Library (1) |
| Ontario County Historical Society (2) | Ontario-Montclair School District (1) |
| OPERA America (1) | Opera Omaha (1) |
| Opera Theatre of St. Louis (1) | Oradell Public Library (1) |
| Oral History Association (1) | Orange County Library System (3) |
| Orange County Public Library (1) | Orange Public Library (1) |
| Orangeburg Library (1) | Orcas Island Historical Museum (1) |
| Ordway Public Library (1) | Oregon Art Institute (2) |
| Oregon Electric Railway Historical Society (1) | Oregon Episcopal School (1) |
| Oregon Historical Society (9) | Oregon Humanities (24) |
| Oregon International Council (5) | Oregon Museum of Science and Industry (3) |
| Oregon Nikkei Legacy Center (1) | Oregon Public Broadcasting (9) |
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| Oregon State University Foundation (1) | Organization of American Historians (5) |
| Organization of American States (2) | Organizing Committee of the VIII Pan American Games (1) |
| Oriental Research Partners (1) | Orion Township Public Library (1) |
| Orland Park Public Library (2) | Orlando Museum of Art (1) |
| Orthodox Church in America (1) | Ortonville Public Library (1) |
| Osage Tribal Museum (1) | Oshkosh Public Museum (1) |
| Oskaloosa Public Library (1) | Osnaburg Local Schools (1) |
| Ossabaw Island Foundation (1) | Ossining Public Library (1) |
| Otherworld Children's Media (2) | Otoe-Missouria Tribe (1) |
| Otter Tail County Historical Museum (1) | Otterbein College (3) |
| Ouachita Baptist University (1) | Ouachita Parish Public Library (2) |
| Our Lady of the Lake University (2) | Ouray Public Library (1) |
| Outagamie County Historical Society, Inc. (2) | Owasso 7th Grade Center (1) |
| Owens Valley Paiute-Shoshone Culture Center (1) | Owensboro Area Museum of Science and History (1) |
| Owsley Brown Frazier Historical Arms Museum Foundation, Inc. (1) | Oxford University Press (4) |
| Oxnard Public Library (2) | Oyster River High School (1) |
