This paper reports on a pilot study of acoustic properties of respiratory sounds in relation to obstructive sleep apnea (OSA). Data were breathing sounds of 60 individuals in three groups of non-, Moderate and Severe OSA using the apnea/hypopnea index (AHI) (20 in each group). The sounds power spectral energy and phase responses were calculated for each respiratory phase, and investigated among the groups. The results of power spectral analysis, congruent with imaging studies' findings, suggest a regional narrowing in upper airway that shows its effect in low and high frequencies as an opposite pattern between the OSA and non-OSA groups. The results of phase analysis also support an increased nonlinearity in high frequencies that is speculated to be related to the regional narrowing and thus regional elasticity of the airway during wakefulness.