This paper proposed an opportunistic network-coded cooperation (ONCC) scheme suitable for multiple unicast transmission pairs in a single-cell wireless network. In contrast to existing work related to network-coded cooperation where the relay always performs network-coding-and-forwarding to help those transmitting terminals, ONCC is opportunistic because the relay decides whether or not to help do cooperation for a source-destination s-d pair based upon limited feedback from the destination. It does not assist a transmission pair unless the feedback indicates failure of the direct transmission between the source and the destination. For a wireless system composed of two s-d pairs and a relay, systematic performance analysis of ONCC was presented in terms of diversity-multiplexing tradeoff and system outage probability. ONCC is proved to be able to achieve an optimal diversity-multiplexing tradeoff which corresponds to the transmit diversity bound in the high-SNR (signal-to-noise ratio) regime. This means that ONCC makes more efficient use of the degrees of freedom of the channel than those protocols where an s-d pair is always be assisted by cooperative communication. Moreover, it shows that under a fixed system energy constraint, ONCC provides a reduced system outage probability while maintaining an increased expected spectral efficiency at the same time compared with incremental relaying protocol, which is similar to ONCC in "opportunistic" property but different from ONCC in that it does not exploit network coding technology.