Players ATPWTA
| Name | Country |
|---|---|
| Colin O'Brien | IRL |
| Dean O'Brien | RSA |
| Christopher O'Connell | AUS |
| David O'hare | IRL |
| Jonny O'Mara | GBR |
| Becker O'Shaughnessey | USA |
| Keelan Oakley | GBR |
| Luke Oakley | GBR |
| Naser Obaidly | KUW |
| Adrian Obert | GER |
| Vladimir Obradovic | SRB |
| Julien Obry | FRA |
| Makoto Ochi | JPN |
| Alvaro Ochoa | COL |
| Andrew Ochotta | CAN |
| Julian Ocleppo | ITA |
| Oleksiy Odazhyu | UKR |
| Wayne Odesnik | USA |
| Adrian Oetzbach | GER |
| Patrick Ofner | AUT |
| Sebastian Ofner | AUT |
| Artem Oganesyan | RUS |
| Dae-Soung Oh | KOR |
| Sang-Oh Oh | KOR |
| Seong-Gook Oh | KOR |
| Ricardo Ojeda Lara | ESP |
| Jules Okala | FRA |
| Issei Okamura | JPN |
| Hiromasa Oku | JPN |
| Guillermo Olaso | ESP |
| Yshai Oliel | ISR |
| Robin Olin | SWE |
| Francisco Olivares | CHI |
| Jose Olivares | DOM |
| Allan Gomes Oliveira | BRA |
| Carlos Oliveira | BRA |
| Goncalo Oliveira | POR |
| Marlon Oliveira | BRA |
| Albano Olivetti | FRA |
| Maxence Olivier | FRA |
| Genaro Alberto Olivieri | ARG |
| Renzo Olivo | ARG |
| Ken Onishi | JPN |
| Julian Onken | GER |
| Masayoshi Ono | JPN |
| Yohei Ono | JPN |
| Ken Onoda | JPN |
| Arata Onozawa | JPN |
| Cosmin Ontica | ROU |
| Reilly Opelka | USA |