In oceanic wireless optical communication systems based on orbital angular momentum multiplexing, oceanic turbulence will bring the channel crosstalk. To solve the problem of channel crosstalk, a spatial diversity mitigation scheme is proposed. Random phase screen method is used to simulate phase perturbation caused by ocean turbulence and split-step transmission method is used to simulate the beam diffraction. Equal gain diversity combination is used to process signal at the receiver. 4-orbital angular momentum-multiplexed channels and 8-spatial diversity are used in the system. The influences of orbital angular momentum mode sets, oceanic turbulence strength and communication distance on the bit-error rate are investigated. Simulation results show that bit error rates decrease obviously with the modes spacing increases from 1 to 2 in the orbital angular momentum multiplexed sets. Performances of bit-error rates remain steady as modes spacing become larger. Bit-error rate will also increase with the increasing transmission distance and turbulence strength. Results show using diversity combination on orbital angular momentum-multiplexed systems can effectively mitigate the performance degradation from oceanic turbulence.