O'Bryant, Kevin: University of California, San Diego. Number theory, logic (specifically nonstandard analysis), probability, ergodic theory, lattices, game theory, and Fourier analysis.
O'Connor, Sean Erik: Free software for primitive polynomial computation, CRC code generation, coding and decoding, parsing theory and cellular automata.
Olson, Luke: Mathematician at Brown University. Research areas include high-order methods and iterative solvers.