Novel OER catalysts - monodisperse Au@Co3O4 core-shell nanocrystals - have been prepared by synthesizing Au nanocrystals, followed by deposition of Co shells and their conversion to Co3O4 shells. Owing to the synergistic effect, Au@Co3O4 nanocrystals have an OER activity 7 times as high as a Au and Co3O4 nanocrystals mixture or Co3O4 nanocrystals alone, and 55 times as high as Au nanocrystals alone.