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In cognitive radio networks, cognitive secondary users (CSUs) can interfere with the primary users (PUs) when they falsely occupy the PU's licensed spectrum. If CSUs in transmission temporarily form a cooperative relaying network, the interference from CSUs to PU can be reduced due to the cooperative diversity. In this paper, the outage performance as well as the achievable diversity order of opportunistic relaying used for cooperative CSUs is investigated. The cooperative relaying network of CSUs is referred to opportunistically relayed cognitive secondary network (ORCSN). Exact outage probabilities and then the diversity order by high SNR approximation are provided under Rayleigh and Nakagami-m fading channels, respectively. It is shown that a spectrum-sensing method used by the CSUs plays an important role in determining the diversity order in ORCSN. Under Rayleigh fading channels, a cooperative sensing enables ORCSN to achieve the full diversity order but a distributed sensing provides the diversity order smaller than one. Under Nakagami-m fading channels, the achievable diversity order is restricted by the Nakagami channel parameters between the PU and the CSUs and also depends on the sensing method. Moreover, the full diversity order can be achieved by a proper selection of the threshold used for spectrum sensing. Numerical investigation is also provided and used to verify the analysis.