This paper analyzes the performance of optimum combining (OC) cooperative interference-limited systems in the presence of co-channel interference (CCI). In particular, we investigate a dual-hop decode-and-forward relaying system comprising a pair of source-destination, multiple relays and multiple interferers affecting the relays as well as the destination. Under this scenario, we first derive the probability density function (PDF) and the cumulative distribution function (CDF) of the signal-to-interference ratio (SIR) at the destination. Based on the results of the CDF and PDF, closed-form expressions for the outage probability and average error probability are derived over Rayleigh fading channels. To investigate the impact of the CCI on the system performance, an asymptotic average error expression is derived to get insights on the diversity and coding gains. Numerical and simulation results are presented to validate our analysis. Results demonstrate that the interference-limited systems achieve the diversity gain of the relaying system.