- O'Brien (pronounced "O'bree-en")
- O'Neill (pronounced "O'neel")
- O'Connor (pronounced "O'kon-er")
- O'Sullivan (pronounced "O'sul-li-van")
- O'Doherty (pronounced "O'doe-er-tee")
- O'Flaherty (pronounced "O'fla-er-tee")
- O'Malley (pronounced "O'mal-lee")
- O'Rafferty (pronounced "O'raf-er-tee")
- O'Reilly (pronounced "O'ree-lee")
- O'Rourke (pronounced "O'roork")