This study analyzes the impact of decode-andforward assisted relay selection algorithms used in cooperative non-orthogonal multiple access (NOMA) systems on the system performance. Two different approaches are considered; traditional maximum-minimum (Max-Min) relay selection solution and two-stage relay selection strategy are investigated. Based on the extensive simulation results, although both relay selection algorithms provide full diversity, the two-stage relay selection method provides almost 3 dB gain for both users. This situation is extremely important in terms of energy efficiency. The bit error rate (BER) performances for the near and far users over the Rayleigh fading channel are evaluated with Monte Carlo simulations, considering the distance between nodes, power allocation coefficient, path loss exponent parameters.