Delirium is a reversible impairment of metabolism in the human brain. Early detection is important, and an effective screening tool for nurses is crucial. The Delirium Observation Screening (DOS) scale is one such screening tool; however, its diagnostic test accuracy has not yet been thoroughly examined. This study, therefore, aimed to evaluate the accuracy of the scale through a systematic review and meta-analysis. In July 2019, a search was conducted in the MEDLINE, CINAHL, Embase, and PsycARTICLES databases, and following a review against pre-defined eligibility criteria, eight studies were finally included. The quality assessment tool of diagnostic accuracy studies was applied to each study and a hierarchical regression model was used to calculate the pooled estimates of sensitivity (90%; 76%–97%, CI 95%) and specificity (92%; 88%–94%, CI 95%). The findings indicated a high diagnostic test accuracy for the DOS scale.