Reactive oxygen species (ROS) formation by atmospheric fine particulate matter (PM) is widely implicated in adverse health effects, yet the molecular determinants of particle-level ROS formation remain poorly constrained, largely due to the chemical complexity of organic aerosol (OA). Here, we link ROS formation to molecular-level OA composition by applying a maximum carbonyl ratio-Van Krevelen framework to ambient fine PM collected from four Chinese megacities, a German semi-urban site, a boreal forest, and laboratory-generated secondary organic aerosol (SOA) from multiple precursors. Using ultrahigh-resolution mass spectrometry combined with hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) and radical measurements, we show the intrinsic ROS (H<sub>2</sub>O<sub>2</sub>+radicals) formation potential of fine PM is governed by OA composition rather than particle mass. Volume-normalized ROS yields increase with fine PM mass concentration and dominate overall exposure in polluted urban environments. In contrast, mass-normalized ROS yields vary independently of particle loading and are strongly driven by the balance between relative fractions of oxidized organic compounds (RF<sub>OOC</sub>) and unsaturated organic compounds (RF<sub>UOC</sub>), i.e., RF<sub>OOC</sub>/RF<sub>UOC</sub>. Across urban sites, mass-normalized ROS and H<sub>2</sub>O<sub>2</sub> yields exhibit strong correlations with RF<sub>OOC</sub>/RF<sub>UOC</sub>, establishing this metric as a robust predictor of ROS formation. Remote, biogenic-influenced aerosols exhibit higher RF<sub>OOC</sub>/RF<sub>UOC</sub> and higher ROS yields per unit mass, whereas urban aerosols exhibit the opposite behavior. Laboratory SOA further supports these composition-reactivity relationships, with OOC-rich biogenic SOA exhibiting the highest ROS yields. This study reveals molecular-level characteristics in governing aerosol ROS formation and introduces a framework for assessing health-relevant aerosol oxidative activity of chemically complex OA.