The use of integrated sensing and communications (ISAC) can improve the performance of both sensing and communication functionalities of wireless networks by flexibly sharing radio resources. However, its performance cannot be always guaranteed due to the dynamic interference circumstances. Therefore, it requires a sophisticated design of cooperation strategies to balance the tradeoff. To provide some useful insights, an application of opportunistic cooperation to ISAC is studied in this letter. First, an opportunistic cooperative scheme of ISAC (OC-ISAC) is proposed, which encourages the sensing tasks reuse radio resources with downlink communications when the transmission requirement can be satisfied. Second, the successful serving probability (SSP) of ISAC is introduced to provide a unified performance evaluation framework for both sensing and communication tasks, and the closed-forms of the SSP are derived for our studied OC-ISAC scheme, which can provide some useful insights on the impacts of interference via asymptotic analysis. Finally, the simulation results are presented to verify the accuracy of the analytical results, and show the performance gains of OC-ISAC scheme.