O
| Ancient name | Location | Modern name | Also known as |
| Oenoe | northern Turkey | Ünye | Oene |
| Oenoe | Argolis, Greece | Oene | |
| Oenoe | Attica, Greece | Oene | |
| Oenus | Laconia, Greece | ||
| Odessos | coastal Ukraine | Odessa | Khadjibey, Khadjibei, Khacdjibei, Hacıbey, Hocabey, Gadzhibei, Chadžibėjus, Hacıbey |
| Odessos | coastal Bulgaria | Varna | |
| Olbia | Sardinia | Civita, Terranova Pausania | |
| Olbia | Ukrainian Black Sea coast | abandoned | Pontic Olbia, Olvia |
| Olous | near Elounda, Crete | drowned | Olus (Ὄλους, Ὄλουλις) |
| Olympia | Elis, Greece | abandoned | Olympí'a (Ολυμπία), Olýmpia (Ολύμπια), Olimpia, Olimbia |
| Olynthus | Chalcidice, northern Greece | destroyed | |
| Oricos | Epirus, near Vlorë, Albania | abandoned | |
| Orestias | Turkish Thrace | abandoned | |
| Oreus | northern Euboea | ||
| Oropus | Attica, Greece | Oropos (Ωρωπός) | |
| Onchesmos | Epirus, southern Albania | Sarandë | Anchiasmos, Άγιοι Σαράντα, Agioi Saranda, Turkish: Aya Sarandi |
Read more about this topic: List Of Ancient Greek Cities