Despite the clinical and prognostic significance of obstructive sleep apnea (OSA) in chronic respiratory diseases (CRDs), there have been few studies about the possible predictors of OSA and the effect of OSA on quality of life in patients with CRDs. The objectives were to identify physiological and clinical parameters that predict the occurrence and severity of OSA and to investigate the effect of OSA on quality of life in patients with CRDs. Seventy-three patients with chronic obstructive pulmonary disease (COPD) and 77 patients with fibrotic interstitial lung disease (ILD) underwent overnight polysomnography (PSG) and pulmonary function testing and completed clinical questionnaires. The oximetry tracing was interpreted blindly with respect to the PSG results. The prevalence of OSA was 44% and 62% in COPD and ILD, respectively. The COPD assessment test item scores related to sleep quality and daily vitality were worse among patients with OSA than among patients without OSA. The STOP-BANG questionnaire (cutoff point ≥ 3) and oxygen desaturation index from the oximetry recording (oxygen desaturation index (ODI) were associated with OSA in CRDs. The STOP-BANG questionnaire with a cutoff point ≥ 3 or 6 had the highest sensitivity and specificity, respectively, in detecting OSA in CRDs. ODI had the best accuracy in identifying OSA and was independently associated with the apnea-hypopnea index in CRDs. We found OSA to be common and associated with worse sleep quality and less daily vitality in patients with advanced CRDs. The STOP-BANG questionnaire with different cutoff points may help rule in or rule out OSA. Overnight oximetry can be used as a screening tool for OSA and can assist the clinical evaluation of OSA in patients with CRDs.