BACKGROUND: Obstructive Sleep Apnoea (OSA) and Chronic Obstructive Pulmonary Disease (COPD) are both independently associated with cognitive impairment. COPD/OSA overlap syndrome could potentially result in greater cognitive impairment that is more than additive. This systematic review evaluates attention, memory, executive function and global cognition in OSA alone compared to COPD/OSA overlap syndrome. METHODS: Systematic searches in MEDLINE, EMBASE, PsycINFO, CINAHL, and CENTRAL identified studies assessing cognitive function in adults with OSA and/or COPD/OSA overlap syndrome. Inclusion criteria required validated diagnostic and cognitive assessment tools. Twelve studies, including 7,424 participants, were reviewed: 10 involving OSA alone and 2 involving overlap syndrome. A narrative synthesis was performed due to methodological heterogeneity. Registration number is: CRD42024557577. RESULTS: OSA alone was primarily associated with mild to moderate cognitive impairment, with attention and executive function most affected, with nocturnal hypoxemia and sleep fragmentation thought to be underlying causative factors. Memory and global cognition were relatively preserved. In contrast, COPD/OSA overlap syndrome was associated with more severe impairments, particularly in memory and global cognition. Overlap patients had significantly lower cognitive scores and a higher prevalence of mild cognitive impairment compared to OSA alone. CONCLUSIONS: Whilst OSA alone is associated with mild to moderate cognitive impairments, COPD/OSA overlap syndrome associates with more pronounced impairments, particularly in memory and global cognition. Nocturnal hypoxemia and systemic inflammation may be important mechanisms. Early cognitive screening and targeted interventions could support clinicians in mitigating these risks.