In this paper, a novel LED power control algorithm is proposed for an OSM based communications system. The proposed algorithm constantly controls the power of the LEDs employed at the transmitter side to provide the minimum overall SER for a mobile user. An exact analytical SER expression is obtained for the general MISO OSM system. It is shown that the proposed algorithm solves a convex optimization problem where linear constraints are used to minimize the SER. Finally, a 2x1 MISO-OSM example is presented to prove the effectiveness of the proposed algorithm. The computer simulations show that the power adaptive MISO-OSM system with the proposed algorithm achieves the average SER of 3.87E-6 for 30 dB SNR.