O Surnames
- O’Brien, Joel Moore
- O’Brien, Oscar Lawrence
- O’Brien, Patrick Henry
- O’Bryan, Harry L.
- O’Bryan, William Wuyts
- O’Connor, James Frederick
- O’Daniel, James Franklin
- O’Donnell, Alfred, M. D.
- Ober, Ezra William
- Offenbacker, David Clark, Hon.
- Oliver, William
- Olson, Nels
- Olson, O. A., Dr.
- Ong, Charles L.
- Orr, David O.
- Osborn, Charles F.
- Ostenberg, Nelse J.
- Oswald, Charley W.
- Otto, Frederick C.
- Overfield, John F.
- Overly, Charles H.
- Owen, Charles
- Owen, William