Background: Liberal oxygen supplementation is often used in acute illness but has, in some studies, been associated with harm. Research Question: The goal of this study was to assess the benefits and harms of higher vs lower oxygenation strategies in acutely ill adults. Study Design and Methods: This study was an updated systematic review with meta-analysis and Trial Sequential Analysis (TSA) of randomized clinical trials. A clear differentiation (separation) was made between a higher (liberal) oxygenation and a lower (conservative) oxygenation strategy and their effects on all-cause mortality, serious adverse events, quality of life, lung injury, sepsis, and cardiovascular events at time points closest to 90 days in acutely ill adults. Results: The study included 50 randomized clinical trials of 21,014 participants; 36 trials with a total of 20,166 participants contributed data to the analyses. Meta-analysis and TSAs showed no difference between higher and lower oxygenation strategies in trials at overall low risk of bias except for blinding: mortality relative risk (RR), 0.98 (95% CI, 0.89-1.09; TSA-adjusted CI, 0.86-1.12; low certainty evidence); serious adverse events RR, 0.99 (95% CI, 0.89-1.12; TSA-adjusted CI, 0.83-1.19; low certainty evidence). The corresponding summary estimates including trials with overall low and high risk of bias showed similar results. No difference was found between higher and lower oxygenation strategies in meta-analyses and TSAs regarding quality of life, lung injury, sepsis, and cardiovascular events (very low certainty evidence). Interpretation: No evidence was found of beneficial or harmful effects of higher vs lower oxygenation strategies in acutely ill adults (low to very low certainty evidence). Clinical Trial Registration: PROSPERO; No.: CRD42017058011; URL: https://www.crd.york.ac.uk/prospero/.