In cold molecular clouds, UV photolysis of icy grain mantles generates radicals that lead to new molecule formation. When radical diffusion is limited by low temperatures, oxygen atom addition and insertion reactions, enabled by photolysis of common ice components such as H2O, CO2, CO, and O3, offer an alternative route to chemical complexity through the production of metastable, highly reactive O(1D) atoms. We examine the reactivity of these oxygen atoms generated by UV photolysis of O3 with methyl cyanide (CH3CN). These studies are conducted in an ultrahigh vacuum chamber at cryogenic and low-pressure conditions equipped with in situ infrared spectroscopy to monitor destruction and product formation in real time. We conclude that oxygen atoms rapidly insert into CH3CN to produce primarily methyl isocyanate (CH3NCO) in matrix free ices. Over the range from 10 K to 40 K, we observe no temperature dependence to either CH3CN destruction or CH3NCO production. When placing CH3CN:O3 in H2O and CO2 ice matrices, we find that CH3NCO formation remains robust, but that the yield likely decreases due to competing reaction pathways. In the case of the H2O ice we also observe a shift in product branching ratios towards alternative pathways such as the formation of hydroxyacetonitrile (HOCH2CN). Overall, our results demonstrate that oxygen atom reactivity provides an important channel for generating chemical complexity from nitriles on cold grains where radical mobility is limited.