| Testator or Main Party | Will Type | Case Number |
| O'Brien, Agnes L. | Prowill | 9652 |
| O'Brien, Bridget E. | Prowill | 10196 |
| O'Brien, Catharine | Prowill | 2749 |
| O'Brien, Christopher | Prowill | 7490 |
| O'Brien, Cornelius F. | Codewill | 22951 |
| O'Brien, John | Prowill | 8945 |
| O'Brien, Margaret | Prowill | 3749 |
| O'Brien, Martin P. | Prowill | 6673 |
| O'Brien, Mary | Prowill | 4313 |
| O'Brien, Michael D. | Prowill | 6488 |
| O'Brien, Nellie | Prowill | 13413 |
| O'Brien, Peter | Prowill | 6453 |
| O'Brien, Sarah | Prowill | 6259 |
| O'Brien, Thomas | Prowill | 3860 |
| O'Connell, Anthony | Prowill | 11897 |
| O'Conner, John J. | Codewill | 11220 |
| O'Connor, Edward J. | Prowill | 9298 |
| O'Connor, Elizabeth | Prowill | 12157 |
| O'Connor, Michael | Codewill | 15658 |
| O'Connor, Stephen | Prowill | 12144 |
| O'Connor, T. J. | Prowill | 10946 |
| O'Dea, Bridget | Prowill | 5182 |
| O'Dea, Patrick | Prowill | 5028 |
| O'Donnel, James | Prowill | 6644 |
| O'Hern, Al F. | Prowill | 11246 |
| O'Keefe, Catherine | Prowill | 5538 |
| O'Meara, Thomas | Prowill | 4989 |
| O'Neil, Louise | Codewill | 21478 |
| O'Neil, Winifred (Bernard) | Prowill | 2940 |
| O'Shaughnessey, Annie | Codewill | 7603 |
| O'Shaughnessy, Patrick | Codewill | 11595 |
| O'Shea, Cornelius | Codewill | 19294 |
| Oakes, William P. | Prowill | 2275 |
| Ochs, Estelle L. | Codewill | 18600 |
| Ochs, Francis | Prowill | 3025 |
| Ochs, Harold Arthur | Codewill | 16524 |
| Ochs, Henrietta | Prowill | 1805 |
| Ochs, Jette | Prowill | 3153 |
| Ochs, Joseph | Prowill | 7726 |
| Ochs, Lippmann | Prowill | 2280 |
| Ochs, Louis | Prowill | 247 |
| Ochs, Louis A. | Prowill | 7230 |
| Ochershausen, George F. | Prowill | 10988 |
| Odell, Mary A. | Prowill | 2842 |
| Odgers, Ella E. | Codewill | 22610 |
| Odgers, John H. | Prowill | 9392 |
| Oelkers, Theodore | Prowill | 7212 |
| Oelkers, Wilhelmine | Prowill | 7538 |
| Oelkers, Wilhelmina | Prowill | 6573 |
| Oerman, William | Prowill | 5022 |
| Oetzmann, Claus | Prowill | 3905 |
| Oetzmann, Jurgen | Prowill | 3568 |
| Offermann, Adolph | Prowill | 13094 |
| Offermann, Dorathea (John) | Prowill | 6879 |
| Ogden, John B. | Prowill | 10693 |
| Ogden, Thomas J. | Codewill | 21317 |
| Ogle, Mary Jane | Prowill | 12283 |
| Ohm, John Frederich | Prowill | 6905 |
| Oldenburg, Dorathea | Prowill | 6722 |
| Oldenburg, Frederick | Prowill | 5086 |
| Oldsen, Hans Nicolaus | Prowill | 5059 |
| Oldsen, Heinrich | Codewill | 10695 |
| Olinger, Carl A. | Codewill | 21795 |
| Oliver, Albourne | Prowill | 2204 |
| Oliver, Margaret Mary | Prowill | 2559 |
| Olsen, Henry | Prowill | 6281 |
| Olshausen, John J. | Prowill | 2973 |
| Oltzen, John J. | Prowill | 6219 |
| Oman, Ernest | Codewill | left blank |
| Orendorff, Amelia | Prowill | 7126 |
| Orendorff, W. J. | Prowill | 8517 |
| Orr, Ann E. | Prowill | 11292 |
| Orth, Anna Louise | Prowill | 930 |
| Osbar, Carl (Charles) | Prowill | 2895 |
| Osbar, Fred | Prowill | 9917 |
| Osbar, Hans | Prowill | 6641 |
| Osbar, Margaretha | Prowill | 2537 |
| Osbar, Ottillie (Hans) | Prowill | 2935 |
| Osborn, George | Prowill | 6726 |
| Osborn, Margaret J. | Codewill | 14318 |
| Osborn, Marietta | Prowill | 8846 |
| Osborn, Nelson L. | Prowill | 6070 |
| Osborn, Lars J. | Prowill | 9632 |
| Oslund, Lars J. | Prowill | 9632 |
| Ott, Catharine | Prowill | 10569 |
| Ott, Edward | Prowill | 13539 |
| Ott, Herman R. | Prowill | 10529 |
| Ott, Hinrich | Prowill | 3032 |
| Otten, John H. | Prowill | 11054 |
| Otten, Joseph | Prowill | 3478 |
| Ottesen, Otto | Prowill | 5806 |
| Otto, Anna Catharina | Prowill | 7309 |
| Otto, Anna Catherine | Codewill | 7309 |
| Otto, Charles A. | Prowill | 5646 |
| Otto, Johann Adolph Edvard | Prowill | 7061 |
| Otto, John A. | Prowill | 2151 |
| Otto, Luisa | Prowill | 9102 |
| Otto, Luisa | Prowill | 9102 |
| Otto, P. Th. Walter | Prowill | 5723 |
| Overdieck, Elise | Prowill | 7663 |
| Overdieck, Gustav L. F. | Prowill | 1391 |
| Owen, Charles | Prowill | 11087 |
| Owen, W. E. | Prowill | 12279 |
| Owens, Hermann | Prowill | 7120 |
| Owens, Thomas J. | Prowill | 8273 |
| Oze, Hans | Prowill | 1594 |
| Ozias, Charity J. | Prowill | 11453 |