Ozonation of drinking and wastewater relies on ozone (O3) and hydroxyl radical (•OH) as oxidants. Both oxidants react with dissolved organic matter (DOM) and alter its composition, but the selectivity of the two oxidants and mechanisms of reactivity with DOM moieties are largely unknown. The reactions of O3 and •OH with two DOM isolates were studied by varying specific ozone doses (0.1-1.3 mg-O3/mg-C) at pH 7. Additionally, conditions that favor O3 (i.e., addition of an •OH scavenger) or •OH (i.e., pH 11) were investigated. Ozonation decreases aromaticity, apparent molecular weight, and electron donating capacity (EDC) of DOM with large changes observed when O3 is the main oxidant (e.g., EDC decreases 63-77% for 1.3 mg-O3/mg-C). Both O3 and •OH react with highly aromatic, reduced formulas detected using high-resolution mass spectrometry (O:C = 0.48 ± 0.12; H:C = 1.06 ± 0.23), while •OH also oxidizes more saturated formulas (H:C = 1.64 ± 0.26). Established reactions between model compounds and O3 (e.g., addition of one to two oxygen atoms) or •OH (e.g., addition of one oxygen atom and decarboxylation) are observed and produce highly oxidized DOM (O:C > 1.0). This study provides molecular-level evidence for the selectivity of O3 as an oxidant within DOM.