The oxidation of methyl(phenyl)silylene, 3f, with molecular oxygen is investigated using matrix isolation spectroscopy in combination with DFT calculations. The UV irradiation (lambda > 305 nm) of phenylsilyldiazomethane, 8, matrix-isolated in argon at 10 K, produces silylene 3f in very high yield. In O-2 doped argon matrixes, the thermal reaction of 3f with O-2 could be directly monitored by IR spectroscopy. The only reaction product is dioxasilirane 5f, presumably formed via cyclization of silanone O-oxide 4f. DFT calculations reveal that the activation barrier for the 4f --> 5f cyclization is less than 1 kcal/mol, too small to allow the matrix isolation of 4f. Dioxasilirane 5f is photolabile, and irradiation with blue light (lambda > 420 nm) results in a [1,2]-phenyl migration to give silaester 7f. The product of the methyl migration, silaester 13, is formed as a minor constituent.