Obstructive Sleep Apnea (OSA) and depression share risk factors and biological features and the two conditions are usually misdiagnosed when they exist together. Therefore, a faster and efficient diagnosis of the comorbidity is needed. Large number of studies have investigated the relationship between OSA and depression, but far fewer studies have focused on the shared biomarkers between the two conditions within signals obtained from sleep recordings. Phase coherence between respiratory sinus arrhythmia (RSA) and respiratory movement has been proposed to be a predicator of slow wave sleep (SWS) in healthy subjects. The aim of this study is to investigate whether this prediction is still valid on OSA patient with and without major depression disorder (MDD) and if it is not valid, how it differs from healthy subjects. Overnight electroencephalogram (EEG), electrocardiograms (ECG) and breathing using plethysmography were recorded from control subjects (20 CONT),OSA subjects with MDD (20 OSAD+) and OSA subjects without MDD (20 OSAD-). Slow wave activity was computed by the amplitude envelope of the EEG δ-wave(0.5–4 Hz). The interbeat intervals (RRI) and respiratory movement were extracted from ECG signals. RRI and respiration were resampled at 10 Hz, and the band passed filtered (0.10–0.4 Hz) before the Hilbert transform was used to extract instantaneous phases of the RSA and respiration. Then the phase coherence(λ) between RSA and respiration were quantified. Using cross-correlation analysis, we found that overnight profiles of λ and δ-wave were correlated in both CONT and OSAD- groups, with significant cross-correlation coefficients (0.441±0.106) and (0.425±0.104) respectively. These correlations were higher than that for OSAD+ group (0.315±0.151). These results suggest that the association between λ and δ-wave has been disturbed in OSAD+ group. Therefore, the correlation between phase coherence (λ) and slow wave sleep can be used as trait marker for distinguishing between depressed and nondepressed OSA patients.