| Last Name | First Name Sort ascending | Year | City | Section/Speaker Type |
|---|---|---|---|---|
| Forster | Otto | 1970 | Nice | Analysis |
| Frostman | Otto | 1936 | Oslo | Analysis |
| Veblen | Oswald | 1928 | Bologna | Plenary |
| Veblen | Oswald | 1936 | Oslo | Plenary |
| Lanford III | Oscar E. | 1974 | Vancouver | Mathematical Physics & Mechanics |
| Randal-Williams | Oscar | 2022 | Virtual | Topology |
| Iyama | Osamu | 2018 | Rio de Janeiro | Algebra |
| Sarig | Omri | 2010 | Hyderabad | Dynamical Systems and Ordinary Differential Equations |
| Reingold | Omer | 2006 | Madrid | Mathematical Aspects of Computer Science |
| Staffans | Olof | 2006 | Madrid | Control Theory and Optimization |
| Biquard | Olivier | 2014 | Seoul | Geometry |
| Schiffmann | Olivier | 2018 | Rio de Janeiro | Lie Theory and Generalizations |
| Wittenberg | Olivier | 2022 | Virtual | Algebraic and Complex Geometry |
| Glenn | Oliver E. | 1932 | Zurich | Mechanics and Mathematical Physics |
| Mathieu | Oliver | 1990 | Kyoto | Lie Groups & Representation |
| Riordan | Oliver | 2010 | Hyderabad | Combinatorics |
| Kharlampovich | Olga | 2014 | Seoul | Algebra |
| Ladyzhenskaya | Olga | 1994 | Zurich | ICM Emmy Noether Lecture |
| Emanouilov | Oleg Yu. | 2006 | Madrid | Control Theory and Optimization |
| Ageev | Oleg N. | 2006 | Madrid | Ordinary Differential Equations and Dynamical Systems |
| Lepski | Oleg | 2022 | Virtual | Statistics and Data Analysis |
| Svensson | Ola | 2022 | Virtual | Mathematics of Computer Science |
| Gabber | Ofer | 2022 | Virtual | Algebraic and Complex Geometry |
| Devisme | Odette | 1932 | Zurich | Analysis |
| Goldreich | Oded | 1994 | Zurich | Mathematical Aspects of Computer Science |