Gas-phase oxidation in a laser-produced plasma is significantly influenced by the availability of oxygen in and around the plume. In this study, we investigate the role of target-derived and ambient oxygen on AlO formation in plasmas generated from aluminum (Al) and Al2O3 targets in air and argon, respectively. Our results highlight that gas-phase oxidation occurs early during the evolution of Al2O3 plasmas in argon, in contrast to Al plasmas in air, where the initial exclusion of oxygen from the plume delays the chemical reactions.