In this letter, we first provide the closed-form exact outage probability of opportunistic single relay selection in decode-and-forward (DF) relaying with the direct source-destination link under arbitrarily distributed Rayleigh fading channels. The signals from the source and the selected relay are combined at the destination by using maximal ratio combining (MRC). We derive the probability density function (PDF) and the cumulative density function (CDF) of received SNR at the destination. Numerical results show that the analytic results exactly match with the simulated ones.