- O'Brien, Michael
- O'Connell, Jack
- O'Connor, Brian
- O'Connor, Patrick
- O'Donnell, Anthony J
- O'Donnell, Pat
- O'Malley, Martin
- O'Neal, Michael
- O'Neil, Daniel
- O'Rourke, Jim
- Oakar, Mary Rose
- Oaks, Nathaniel T
- Obama, Barack
- Oberstar, James L
- Obey, David R
- Obrador, Andres Manuel Lopez
- Obuchowski, Michael
- Oder, G. Glenn
- Olmert, Ehud
- Olson, Donald
- Olson, Mark
- Oppenheimer, Suzi
- Oropeza, Jenny
- Orszag, Peter
- Ortiz, Deborah
- Ortiz, Felix
- Osborne, Tom
- Ose, Doug
- Osterman, Harry
- Owings III, George W.
- Oxley, Michael G
