Density functional theory is applied to the study of the oxygen atom transfer reaction from sulfoxide (DMSO) to phosphine (PMe3) catalyzed by the [MoO2](2+) active core. In this work, two fundamentally different roles are explored for this dioxometal complex in the first step of the catalytic cycle: as an oxidizing agent and as a Lewis acid. The latter turns out to be the favored pathway for the oxygen atom transfer. This finding may have more general implications for similar reactions catalyzed by the same [MoO2](2+) core.