In this paper, we study the outage probability (OP) of the cooperative underlay cognitive radio (CR) networks consisted of a secondary relay node and K non-orthogonal multiple access (NOMA) secondary destination users (SDUs) under imperfect channel state information (CSI) conditions. We first derive generalized closed-form expressions for the outage probability (OP) of NOMA-enabled SDUs. The numerical results show that the considered system model outperforms cooperative orthogonal multiple access in terms of the OP. Finally, the obtained analytical expressions are corroborated by Monte Carlo simulations.