The oxygenation history of Earth's atmosphere and oceans has been divided into five stages. Stage 1 (3.85–2.45 Ga) saw low atmospheric and oceanic oxygen levels, with possible oxygen oases in shallow oceans. Stage 2 (2.45–1.85 Ga) marked a rise in atmospheric oxygen to 0.02–0.04 atm, with shallow oceans becoming mildly oxygenated while deep oceans remained anoxic. Stage 3 (1.85–0.85 Ga) was relatively stable, with little change in atmospheric oxygen levels, and both shallow and deep oceans were mildly oxygenated. Stage 4 (0.85–0.54 Ga) saw a rise in atmospheric oxygen to nearly 0.2 atm, with shallow oceans oxygenated and deep oceans anoxic during ice ages. Stage 5 (0.54 Ga–present) saw a peak in atmospheric oxygen during the Carboniferous before returning to current levels, with shallow oceans oxygenated and deep oceans fluctuating in oxygenation. Key factors influencing oxygenation include the evolution of cyanobacteria, changes in ocean circulation, and the burial of organic matter. The Great Oxidation Event (GOE) around 2.4–2.0 Ga was a major transition, driven by the oxidation of iron and sulfur, and the increase in atmospheric oxygen. The "boring billion" (1.85–0.85 Ga) saw relatively stable oxygen levels, with mild oxygenation in deep oceans. The Neoproterozoic era (0.85–0.54 Ga) saw significant oxygenation, with the rise of marine life and the formation of phosphorite deposits. The Phanerozoic era (0.54 Ga–present) saw fluctuations in oceanic oxygen levels, with periods of anoxia such as the Cretaceous Oceanic Anoxic Events (OAEs). Overall, the oxygenation of the atmosphere and oceans has been a complex process influenced by biological, geological, and climatic factors.The oxygenation history of Earth's atmosphere and oceans has been divided into five stages. Stage 1 (3.85–2.45 Ga) saw low atmospheric and oceanic oxygen levels, with possible oxygen oases in shallow oceans. Stage 2 (2.45–1.85 Ga) marked a rise in atmospheric oxygen to 0.02–0.04 atm, with shallow oceans becoming mildly oxygenated while deep oceans remained anoxic. Stage 3 (1.85–0.85 Ga) was relatively stable, with little change in atmospheric oxygen levels, and both shallow and deep oceans were mildly oxygenated. Stage 4 (0.85–0.54 Ga) saw a rise in atmospheric oxygen to nearly 0.2 atm, with shallow oceans oxygenated and deep oceans anoxic during ice ages. Stage 5 (0.54 Ga–present) saw a peak in atmospheric oxygen during the Carboniferous before returning to current levels, with shallow oceans oxygenated and deep oceans fluctuating in oxygenation. Key factors influencing oxygenation include the evolution of cyanobacteria, changes in ocean circulation, and the burial of organic matter. The Great Oxidation Event (GOE) around 2.4–2.0 Ga was a major transition, driven by the oxidation of iron and sulfur, and the increase in atmospheric oxygen. The "boring billion" (1.85–0.85 Ga) saw relatively stable oxygen levels, with mild oxygenation in deep oceans. The Neoproterozoic era (0.85–0.54 Ga) saw significant oxygenation, with the rise of marine life and the formation of phosphorite deposits. The Phanerozoic era (0.54 Ga–present) saw fluctuations in oceanic oxygen levels, with periods of anoxia such as the Cretaceous Oceanic Anoxic Events (OAEs). Overall, the oxygenation of the atmosphere and oceans has been a complex process influenced by biological, geological, and climatic factors.