This meta-analysis set out to ascertain the cognitive function of obstructive sleep apnea (OSA) patients as measured through objective neuropsychological tests. The meta-analysis investigated the cognitive functioning of these patients prior to them receiving any treatment such as continuous positive airway pressure (CPAP). A total of 19 studies met the study inclusion criteria. Results revealed statistically significant negative effect sizes in the cognitive domains of non-verbal memory, concept formation, psychomotor speed, construction, executive functioning, perception, motor control and performance, attention, speed of processing, working and verbal memory, verbal functioning and verbal reasoning. The clinical implication of these results, the possible causal mechanisms of the cognitive impairments and the implication of these for future research were each discussed. Despite a number of important limitations, the analysis highlights the need for clinicians to comprehensively explore complaints about sleep disturbance, particularly OSA, in all clinical assessments to ensure control for this important confounder in order to ensure appropriate attribution of the source of any observed cognitive compromise.