- O'Brien
- O'Connor
- O'Donohue
- O'Farrell
- O'Keefe
- O'Meehan
- O'Riagain
- O'Sullivan
- Ochterlony
- Ogilvie (Ogilvy)
- Ogilvie (Ogilvy) Hunting
- Ogilvie (Ogilvy) Hunting Ancient
- Ogilvie (Ogilvy) Hunting Modern
- Ogilvie (Ogilvy) of Strathallan
- Oliphant
- Oliphant Ancient
- Oliphant Modern
- Oliver
- Orr
- Orrock
- Outlander Fraser
- Offaly
- Owen of Wales