| Last Name Sort descending | First Name | Year | City | Section/Speaker Type |
|---|---|---|---|---|
| Oh | Hee | 2010 | Hyderabad | Lie Theory and Generalisations |
| Oh | Yong-Geun | 2006 | Madrid | Geometry |
| Ohsawa | Takeo | 1990 | Kyoto | Real & Complex Analysis |
| Ohtsuki | Tomotada | 1998 | Berlin | Topology |
| Okamoto | Hisashi | 1998 | Berlin | Numerical Analysis & Scientific Computing |
| Okounkov | Andrei | 2006 | Madrid | Probability and Statistics/Prize winner |
| Okounkov | Andrei | 2018 | Rio de Janeiro | Plenary |
| Oldenburger | Rufus | 1936 | Oslo | Algebra and Number Theory |
| Oleinik | O. A. | 1970 | Nice | Analysis |
| Olevskii | A. M. | 1986 | Berkeley | Real and Functional Analysis |
| Oliver | Bob | 1998 | Berlin | Topology |
| Olsanskii | A. Yu | 1983 | Warszawa | Algebra |
| Olshanski | Grigori | 2014 | Seoul | Combinatorics |
| Omara | M. A. | 1936 | Oslo | Rational and Applied Mechanics |
| Onicescu | O. | 1928 | Bologna | Analysis |
| Oniecescu | Octav | 1936 | Oslo | Probability Calculus, Mathematical Statistics, Insurance Mathematics, and Econometry |
| Ono | Kaoru | 2006 | Madrid | Topology |
| Ono | T. | 1966 | Moskva | Algebraic geometry & complex manifolds |
| Opdam | E.M. | 2006 | Madrid | Lie Groups and Lie Algebras |
| Ore | Øystein | 1924 | Toronto | Algebra, Theory of Numbers, Analysis |
| Ore | Øystein | 1928 | Bologna | Analysis |
| Ore | Øystein | 1932 | Zurich | Algebra and Number Theory |
| Ore | Øystein | 1936 | Oslo | Plenary |
| Orlando | L. | 1908 | Rome | Arithmetic, Algebra, Analysis |
| Orlicz | W. | 1936 | Oslo | Analysis |