Background In chronic obstructive pulmonary disease (COPD)–obstructive sleep apnea (OSA) overlap syndrome, the apnea–hypopnea index (AHI) confirms and grades OSA but does not fully explain nocturnal hypoxemia or carbon dioxide retention. Objective We propose a post-diagnostic, mechanism-informed framework for interpreting nocturnal oxygenation and ventilation after both COPD and OSA have been objectively confirmed. Methods This targeted narrative review synthesized PubMed literature and major guideline sources through 20 May 2026. Reporting followed established narrative-review quality principles, and evidence use was categorized as guideline-supported, direct overlap-syndrome observational evidence, indirect COPD or OSA evidence, or physiological inference. Results The framework separates three baseline mechanisms: event-related upper-airway obstruction, sustained low-baseline oxygenation or gas-exchange impairment, and sleep-related hypoventilation or chronic hypercapnia. Residual hypoxemia after optimized positive airway pressure or noninvasive ventilation is treated as a post-treatment reassessment scenario rather than a baseline phenotype. Suggested core reporting includes the AHI measurement method, hypopnea rule, oxygen desaturation threshold, time spent below specified oxygen saturation thresholds, mean and verified minimum oxygen saturation, rapid eye movement and supine exposure, treatment context, oximetry settings, and carbon dioxide monitoring when indicated. Conclusion AHI remains essential for diagnosing OSA, but oxygenation and ventilation in overlap syndrome may be better interpreted by mechanism. The proposed framework is an interpretive and reporting scaffold rather than a treatment algorithm, and its clinical utility, reproducibility, and prognostic value require prospective validation in well-characterized cohorts.