Based on decode-and-forward (DF) protocol, this work focuses on the adaptive power allocation and outage performance of underlay cognitive radio and opportunistic relaying (UCR-OR) systems with direct path between cognitive source and destination. The UCR-OR systems suffer from the interference of multiple primary user (PU) pairs. Under the outage constraint of PUs and the cognitive peak transmit power limit, we first obtain the adaptive power allocation schemes for secondary transmitters. Secondly, we obtain the exact closed-form expression to the outage probability of UCR-OR systems by using appropriate mathematical proof. Finally, to obtain a clear insight and to highlight the effect of system parameters on the performance of UCR-OR systems, the asymptotic closed-form expression of outage probability is achieved with the assumption of high cognitive transmit power. The presented simulations show that, due to the adaptive power allocation employed, the outage probability of UCR-OR systems is decreasing with PUs' transmit power Pp when Pp is less than a specific value P � . Only when the value of Pp is