<p>Earth’s atmosphere underwent permanent oxidation during the Great Oxidation Event approximately 2.45–2.22 billion years ago (Ga) due to excess oxygen (O₂) generated by marine cyanobacteria. However, understanding the timing and tempo of seawater oxygenation before the Great Oxidation Event has been hindered by the absence of sensitive tracers. Nitrogen (N) isotopes can be an indicator of marine oxygenation. Here we present an ~200 Myr nitrogen isotope oscillation recorded by Neoarchaean and Palaeoproterozoic banded iron formations from the Hamersley Basin, Western Australia, that were deposited in relatively deep marine shelf environments. Paired with the Jeerinah Formation shale record, our data from the Marra Mamba Iron Formation suggest that oxic conditions expanded to banded iron formation depositional environments from ~2.63 to 2.60 Ga. Subsequently, a positive δ¹⁵N excursion occurred in the ~2.48 Ga Dale Gorge Member, marking a decline in seawater O₂ and enhanced denitrification. This O₂ deficit was followed by a second phase of increasing O₂ levels as indicated by a gradual return to moderately positive δ¹⁵N values in the ~2.46 Ga Joffre Member and 2.45 Ga Weeli Wolli Iron Formation. These variations underscore a nonlinear history of marine oxygenation and reveal a previously unrecognized oscillation in seawater O₂ levels preceding the Great Oxidation Event.</p>