The evolution of vascular plants and their spread across the land surface, beginning ∼420 Ma, progressively increased the rate of weathering of phosphorus from rocks. This phosphorus supply promoted terrestrial and marine productivity and the burial of organic carbon, which has been the major source of O2 over geological timescales. Hence, it is predicted that the rise of plants led to an increase in the O2 content of the atmosphere from ∼12 vol %, 570–400 Ma to its present level of ∼21 vol % by ∼340 Ma. Previous modelling studies suggest that O2 then rose to ∼35 vol % ∼300 Ma. Such high concentrations are difficult to reconcile with the known persistence of forests, because rising O2 increases the frequency and intensity of vegetation fires, tending to decrease biomass and cause ecological shifts toward faster regenerating ecosystems. Rising O2 also directly inhibits C3 photosynthetic carbon assimilation and increases the production of toxic reactive oxygen species in cells. These effects suppress plant-induced phosphorus weathering and hence organic carbon burial, providing a sensitive negative feedback on O2. A revised model predicts that this mechanism could have regulated atmospheric O2 within the range 15–25 vol % for the last 350 million years.