Orthogonal Time Frequency Space (OTFS) is a promising approach which is widely employed in sixth generation (6G) wireless network systems. Because of its superior performance in high-mobility environments, OTFS modulation has received a lot of attention lately. Due to OTFS modulation works in the delay-Doppler (DD) domain rather than the conventional time-frequency (TF) domain, it works effectively in such circumstances. The idea of non-orthogonal multiple access (NOMA) is integrated into OTFS as an important approach to improve the spectral efficiency (SE) to investigate the efficiency potential and performance. In this research, we study OTFS modulated NOMA system for two destination users in the high mobility environment. The message passing detection algorithm is utilized to examine bit error rate (BER) performance for both near and far users in the proposed OTFS modulated NOMA system. The BER simulation results demonstrate that the power allocation (PA) coefficient, delays, and Doppler effects significantly impact the performance of the system. It is observed that the performance of the far user did not drop below a particular BER level. The BER value for the 𝑈1 user is 0.1, while the BER value for the 𝑈2 user is nearly 0.25 at 10 dB SNR, resulting in a 2.5 times better BER performance in the 4-QAM scenario. The BER value is about 0.27 for the 𝑈1 user, while the BER value for the 𝑈2 user is approximately 0.33 at 10 dB SNR in the 16-QAM approach. It is concluded that the Doppler effect causes BER performance degradation for both users.