- O'Brien, Daniel
- O'Brien, Daniel Doyle
- O'Brien, Edward Louis
- O'Brien, Joel Moore
- O'Brien, Matthew T.
- O'Brien, Oscar Lawrence
- O'Brien, Patrick
- O'Brien, Patrick Henry
- O'Bryan, Harry L.
- O'Bryan, William Wuyts
- O'Bryant, John H.
- O'Connell, J. Harvey
- O'Connor, James Frederick
- O'Daniel, Aaron Virgil
- O'Daniel, James Franklin
- O'Day, John, Esq.
- O'Donnell, Alfred, M. D.
- O'Fallon, Charles Pope
- O'Fallon, John Julius
- O'Hearn, William A., Hon.
- O'Laughlin, James
- O'Malley, Patrick, Hon.
- O'Neil, Edward Thomas
- Oak, Jonathan
- Oak, Nathaniel
- Oaks, Jonathan
- Oaks, Nathan
- Oaks, Nathan
- Oaks, Thaddeus
- Oaks, William A.
- Oatman, Harrison B.
- Oatman, Louis J. M. D.
- Ober, Amasa
- Ober, Benjamin
- Ober, Ezra William
- Oden, John
- Odle, Marvin Mrs.
- Odle, Mrs. Grover C.
- Odom, Robert
- Oehler, Emanuel F. M.D.
- Offenbacker, David Clark, Hon.
- Ogden, D. T. Mrs.
- Ogilvie, George
- Ohms, Albert
- Oldenburg, Ludwick
- Oldham, Ira B. M. D.
- Oliver, Arthur Lee
- Oliver, Elijah W.
- Oliver, Hiram W.
- Oliver, Hiram W.
- Oliver, Turner
- Oliver, William
- Olmstead, Hannah J., Mrs.
- Olmstead, Seth
- Olmsted, Judge M. L.
- Olmsted, Percy Napier, Ensign
- Olmsted, Robert Ward
- Olney, John E.
- Olson, Nels
- Olson, O. A., Dr.
- Ong, Charles L.
- Orcutt, Obert Lee
- Orr, David O.
- Orvis, Lorin
- Osborn, Charles F.
- Osborne, William Bushnell
- Osburn, Henry C.
- Osburn, John
- Osgood, Addison Newton, Hon.
- Osgood, Burrus
- Osgood, Carlos Pierce
- Osgood, Charles Henry
- Osgood, Elihu
- Osgood, Orren
- Ostenberg, Nelse J.
- Ostrom, Dr. Louis
- Oswald, Charley W.
- Otterson, Thomas J.
- Ottley, G. Lynn
- Ottley, George W.
- Ottley, Thomas
- Ottley, William
- Otto, Frederick C.
- Ouimette, Esdras N.
- Overacre, John W.
- Overfield, John F.
- Overly, Charles H.
- Owen, Thomas H. Mrs.
- Owen, Charles
- Owen, Ferguson
- Owen, William
- Owens, Alexander
- Owens, James W. F.
- Owens, Thomas
- Owens-Adair, Bethina Angelina
- Owings, George
- Ozmont, G. W.