| O |
| Name | Number of records |
| Oak Tree Publications, Inc. | 377 |
| Oceana Publications, Inc. | 1099 |
| OCLC Online Computer Library Center, Inc. | 1875 |
| October Films, Inc. | 760 |
| Odyssey Group Video | 781 |
| Off Backstreet Music | 96 |
| Ohio | 604 |
| Ohio State University Press | 510 |
| Ohio University Press | 778 |
| Oklahoma | 192 |
| Oklahoma State University. Board of Regents | 279 |
| Oklahoma State University, Stillwater. Board of Regents | 128 |
| Old Crow Music | 123 |
| Old Friends Music | 281 |
| Omaha Creative Group, Inc. | 725 |
| Omnigraphics, Inc. | 438 |
| One Four Three Music | 122 |
| One Two Hit Publishing | 227 |
| Open Court Publishing Company | 940 |
| Opryland Music Group | 139 |
| Options Publishing, Inc. | 374 |
| Orbis Books | 358 |
| Orca Songs | 84 |
| Order of Saint Benedict, Inc. (Collegeville, MN) | 241 |
| Organized Noize | 109 |
| Oriental Trading Company, Inc. | 9990 |
| Orion Pictures Corporation | 696 |
| Orion Pictures Corporation. | 1613 |
| Orion Pictures Corporation. Orion Television Corporation. | 177 |
| Oryx Press | 683 |
| Osho | 331 |
| O-Tex Music | 252 |
| Ottenheimer Publishers, Inc. | 859 |
| Our Sunday Visitor, Inc. | 320 |
| Our Sunday Visitor Publishing | 320 |
| Out of Pocket Productions, Ltd. | 128 |
| Overdue Music | 86 |
| Overseas Filmgroup, Inc. | 1395 |
| Oxford University Press | 2178 |
| Oxford University Press, Inc. | 5736 |
| Oxmoor House, Inc. | 850 |
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