Earth's surface underwent stepwise oxygenation before persistently reaching modern levels late in its history1, 2, 3, 4-5, but the details of this transition remain unclear5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15-16. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Delta ' 17Osulfate), a proxy linked to the atmospheric partial pressure of O2 (pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}_{{{\rm{O}}}_{2}}$$\end{document})17, 18-19. This record, together with existing sedimentary Delta 33S data20, 21-22, demonstrates a 2-Gyr transition characterized by generally low, fluctuating pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}_{{{\rm{O}}}_{2}}$$\end{document} between an O2-free state before 2.4 billion years ago (Ga) and a modern pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}_{{{\rm{O}}}_{2}}$$\end{document} state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Delta ' 17Osulfate and sulfate-delta 34S during major negative carbonate-delta 13C excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}_{{{\rm{O}}}_{2}}$$\end{document}, which may have driven episodic ocean oxygenation through an increased atmospheric O2 influx. This process intensified the oxidation of marine organics and reduced-sulfur species, while triggering temporary pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${p}_{{{\rm{O}}}_{2}}$$\end{document} drawdowns as negative feedback15. These findings support a dynamic, lengthy co-oxygenation history for the atmosphere and oceans-marked by long-term positive coupling and short-term negative feedbacks-offering a coherent explanation for the anomalous Neoproterozoic carbon cycles23,24 and the protracted, episodic rise of complex life25, 26-27.