High-valent metal-oxo species are typical catalytic cycle intermediates in mono-oxygenases and dioxygenases and commonly react through oxygen atom transfer to substrates. In this work we study a biomimetic model complex with a 1,1'-bis((3,5-dimethylpyridin-2-yl)methyl)-2,2'-bipiperidine ligand system bound to a manganese(IV)-oxo(hydroxo) species and study its formation from manganese(II)-hydroxo and H2O2 as well as its reaction with (S)-1-phenylisochromane through dehydrogenative α-oxygenation. The work utilizes density functional theory methods to explore its catalytic cycle and its reactivity patterns. We show that the manganese(IV)-oxo(hydroxo) species is an active oxidant and preferentially the oxo group abstracts a hydrogen atom from substrate with barriers well lower in energy than those found for hydrogen atom abstraction by the hydroxo group. Interestingly, the rate-determining step is the OH rebound rather than the hydrogen atom abstraction, which would imply they would have limited kinetic isotope effect for the replacement of the transferring hydrogen atom by deuterium.