Eamonn Anthony O'Brien FRSNZ is a professor of mathematics at the University of Auckland, New Zealand, known for his work in computational group theory and p-groups.

His dissertation, The Groups of Order Dividing 256, was supervised by Michael F.

[1] O'Brien's early work concerned classification, up to isomorphism, of groups of order 256.

This led to classifications of many further families of small order groups.

These classifications comprise the tables provided in the computer algebra systems SageMath, GAP, and Magma.

For a 20-year span from the mid-1990s, O'Brien led the so-called Matrix Group Recognition Project whose primary objective is to solve the following problem: given a list of invertible matrices over a finite field, determine the composition series of the group.