Cooperative relaying is a method used mainly to improve cellular networks in terms of diversity gain and coverage extension. However, its performance is gravely affected by the co-channel interference (CCI), especially when a high channel reuse is needed. Optimum combining (OC) is the combining technique that maximizes the signal-to-interference-plus-noise-ratio (SINR) and eliminates the CCI, achieving the best diversity gain. In the present paper, the performance analysis of cooperative amplify-and-forward relaying by using OC is extended for a scenario with multiple co-channel interferers, multiple-antenna relays and multiple antenna-destination. Also, the techniques of multiple relay transmission (MRT) and relay selection with transmit antenna selection (RS-TAS) are evaluated and compared. An approximation of the SINR and the diversity gain are obtained for both transmission techniques. The moment-generating function (MGF) of SINR is derived in order to obtain the average bit-error rate (BER) of the system. Analytical results are validated using Monte-Carlo simulations, which shows that the proposed scenario combats the CCI in a non obvious way and obtains diversity gain in both transmission techniques. Moreover, the MRT technique cancels a greater number of interferers than RS-TAS, while RS-TAS achieves better spectral efficiency and obtains better performance for small number of interferers.