Players ATPWTA
| Name | Country |
|---|---|
| Carlos Oliveira | BRA |
| Goncalo Oliveira | POR |
| Marlon Oliveira | BRA |
| Albano Olivetti | FRA |
| Maxence Olivier | FRA |
| Genaro Alberto Olivieri | ARG |
| Renzo Olivo | ARG |
| Gauthier Onclin | BEL |
| Ken Onishi | JPN |
| Julian Onken | GER |
| Masayoshi Ono | JPN |
| Yohei Ono | JPN |
| Ken Onoda | JPN |
| Arata Onozawa | JPN |
| Cosmin Ontica | ROU |
| Reilly Opelka | USA |
| Giacomo Oradini | ITA |
| Jakub Oravec | SVK |
| Cristian Orduna-Guzman | MEX |
| Junior A. Ore | USA |
| David Oringa | UGA |
| Elliott Orkin | USA |
| Vladyslav Orlov | UKR |
| Fabrizio Ornago | ITA |
| Philip Orno | DEN |
| Eduardo Yahir Orozco Rangel | MEX |
| Brian Orradre | ARG |
| Alejandro Ortega Palacios | ESP |
| Roberto Ortega-Olmedo | ESP |
| Adrian Ortiz | MEX |
| Juan Pablo Ortiz | ARG |
| Oscar Gabriel Ortiz | ARG |
| Myles Orton | GBR |
| Mazen Osama | EGY |
| Hikaru Oshiro | JPN |
| Marko Osmakcic | SUI |
| Guilherme Osorio | BRA |
| Philipp Oswald | AUT |
| Yuma Otomo | JPN |
| Oscar Otte | GER |
| Federico Ottolini | ITA |
| Thomas Otu | NGR |
| Lamine Ouahab | MAR |
| Josselin Ouanna | FRA |
| Gueninle Abdoul Kari Ouattara | CIV |
| Greg Ouellette | USA |
| Victor Ouvrard | FRA |
| Bowen Ouyang | CHN |
| Alexander Ovcharov | RUS |
| Orel Ovil | ISR |