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In this letter, we study outage performance of opportunistic single relay selection (OSRS) in decode-and-forward (DF) relaying with unequal-power co-channel interferers under Rayleigh fading channels. With the interferers, signal-to-interference-plus-noise ratio (SINR) may not be so great when the interferers also transmit signals at a power level similar to the source. In this case, high signal-to-noise ratio (SNR) approximation often used in outage analysis is not suitable for giving an adequate outage expression. We provide an exact and closed-form outage expression. And using asymptotic analysis, we show that OSRS still achieves full diversity gain in the presence of a finite number of interferers whose transmission powers are finite. When a finite number of interferers also transmit signals at the power level proportional to the source, we show that the asymptotic outage decreases log-linearly as the density of nodes increases. Finally, we show that when the number of interferers is proportional to the node density, the higher density, though creating the greater number of potential relays, does not necessarily contribute to improving the outage performance but deteriorates the performance.