Cooperative relaying achieves a wide range of advantages including diversity gain, coverage extension and mitigation of shadowing. Nevertheless, the performance advantages of cooperative relaying diminish considerably if co-channel interference is present. Optimum combining (OC) can be used to mitigate the adverse effects of co-channel interference in wireless communications. The performance of optimum combining in a decode-and-forward relay network with N equal-power interferers is analyzed. The probability density function of the output signal-to-interference-plus-noise ratio is obtained and a closed-form expression for the exact outage probability is derived for N ≥ M + 1 where M is the number of relay nodes. An approximation for the symbol error rate (SER) is presented. The performance results and closed-form expressions for outage probability and for SER suggest that the asymptotic diversity gain of OC is equal to M, which is a significant improvement over maximal-ratio combining, whose asymptotic diversity gain is equal to zero when operating in co-channel interference.