Unmanned aerial vehicles (UAVs) will constitute an important component of 6G mobile networks to improve coverage in scenarios where deployment of terrestrial infrastructure is challenging or economically infeasible. In this paper, we study a non-orthogonal multiple access (NOMA)-based cooperative UAV network in generic scattering environments. In this system, NOMA improves overall throughput and cooperative communication using decode-and-forward relaying at the UAVs improves coverage. A semi-analytical model of the Rician K-factor is utilized to assess the impact of different scattering environments on the outage probability of the considered UAV network as a function of frequency. Numerical results are provided to reveal the effect of frequency, number of scatterers, and radius of the scattering sphere on the outage probability.