We propose a subchannel allocation by considering multiuser diversity to reduce Self Interference (SI) which arises due to the destrunction of orthogonality of spreading codes in Orthogonal Frequency Division Multiple Access - Code Division Multiplexing (OFDMA-CDM) system. The conventional OFDMA-CDM does not effectively reduce SI, since it does not consider multiuser diversity. The proposed scheme allocates subchannels to the user who has largest minimum Carrier to Interference Ratio (CIR) since the conventional weight combining is done by minimum channel gain. By doing the proposed subchannel allocation, minimum CIR becomes high and each user can use good subchannels. Due to the effective multiuser diversity, demodulated symbols can be weighted by high channel gain. Since the proposed scheme can generate reliable demodulated symbols, the proposed scheme can reduce SI. The simulation results show that the proposed scheme can improve the BER performance.