OSA is characterized by repetitive collapses of the upper airways during sleep. Computational fluid dynamics can be used to investigate the abnormal pressure distribution in the patient’s airways. The computational costs and model reconstruction effort can be reduced by focusing the simulations on the pharynx and replacing the nasal cavity by a simple pipe structure. In this work, the effects of the mentioned replacement on the simulated flow are evaluated. Airflow simulations using the k-ω turbulence model are performed in the anatomically correct airway of a patient having a high difference in the inspiratory volume flow rates of both nostrils, as well as in a model with replaced nasal cavity by a simple pipe structure. The simulated pressure distributions of both models are in very good agreement indicating the acceptability of replacing the nasal cavity by simple pipe structures in in-silico airflow analyses of OSA patients.