In the gas phase, neutral ozone (O3) transfers an oxygen atom to several positive ions, i.e. the radical cations of pyridines (R−Py+•; R = H, CH3, C2H5, and Cl), pyrimidine (Pi+•), and alkyl halides (CH3X+•; X = Cl and I), and the halogen cations (X+; X = Cl, Br, and I). Reactivity changes drastically within the halogen series (Cl+ ≪ Br+ ≤ I+), whereas no O-transfer occurs to F+. The oxide derivatives R−Py+−O•, Pi+−O•, CH3X+−O•, and XO+ are formed, as demonstrated by pentaquadrupole (QqQqQ) double- and triple-stage mass spectrometry. No oxygen atom transfer occurs, however, in “inverse” reactions, i.e., those of ionized ozone (O3+•) with the corresponding neutrals; and charge transfer dominates. Ab initio calculations suggest that O-transfer from ozone to ionized pyridine yields ionized pyridine N-oxide via simple nucleophilic addition of ozone as opposed to 1,3-dipolar cycloaddition. Similar nucleophilic addition followed by O2 loss is also the most likely mechanism for O-transfer from ozone to the ionized alkyl halides and halogen cations. This novel O-transfer reaction to positive ions, which expands our knowledge of the rich chemistry of ozone, introduces a new pathway for the gas-phase oxidation of halogen atoms, pyridines, pyrimidines, alkyl halides, and analogues, and consequently for the gas-phase generation of their chemically interesting but difficult to access ionized oxides.