| O’Brien, John |
| O’Brien, Mary |
| O’Brien, Patrick Dennis (minor) |
| O’Connell, William R. |
| O’Conner, James |
| O’Connor, Ellen (minor) |
| O’Connor, John Sr. |
| O’Day, Nellie |
| O’Dell, C. H. |
| O’Donnell, Jeremiah |
| O’Gorman, William |
| O’Hanly, Robert E. |
| O’Neal, Mary A. |
| O’Neil, Henry |
| O’Neill, James |
| O’Roark, Strouther |
| Oameg/Omeg, Christopher |
| Oathout, Marion E. (minor) |
| Oathout, William |
| Oberholtzer, H. H. |
| Oblinger, H. G. P. |
| Officer, Frank W. (minor) |
| Officer, Mary A. |
| Officer, Robert |
| Officer, Thomas |
| Ohlendorf, Grace |
| Ohlendorf, Julia C. |
| Olcott, George |
| Older, Margaret |
| Oldham, Ella & Emma (minors) |
| Oldham, Olive G. |
| Oliver, Isabella R. |
| Oliver, John T. |
| Oliver, William G. |
| Olsen, Christian Peter |
| Olsen, Hans |
| Olsen, Herman & Adolph (minors) |
| Olsen, Jens |
| Olsen, Mary |
| Olsen, Max |
| Olsen, Mike |
| Olsen, Nels |
| Olsen, Niels |
| Olsen, Niels |
| Olsen, Paul |
| Olsen, Peter Sr. |
| Olson, Amanda Sundgren |
| Olson, Charles |
| Oltman, Albert (minor) |
| Oltman, John M. |
| Oltman, Mehno Adolph |
| Oltmanns, Adolph |
| Oltmans (minor) |
| Orme, Sarah Anna |
| Orr, Lamont |
| Orr, William |
| Osborn, Charles |
| Osborn, Harrison V. |
| Osborn, Henry Dr. |
| Osborn, J. W. |
| Osborn, Lucy, James, Edwin, Charles (minor) |
| Osborn, Major Curtis (minor) |
| Oskows, Christina |
| Osler, Jesse |
| Osler, Orman |
| Ostrom, John |
| Otto, Christian O. |
| Otto, James |
| Ouren, Francis W. |
| Ouren, Hogan |
| Ouren, John E. |
| Ouren, Louis H. & Edward M. (minors) |
| Overton, Americus |
| Overton, Cortez F. |
