Oxidative O-2-dependent biotransformations are promising for chemical synthesis, but their development to an efficiency required in fine chemical manufacturing has proven difficult. General problem for process engineering of these systems is that thermodynamic and kinetic limitations on supplying O-2 to the enzymatic reaction combine to create a complex bottleneck on conversion efficiency. We show here that continuous-flow microreactor technology offers a comprehensive solution. It does so by expanding the process window to the medium pressure range (here, <= 34 bar) and thus enables biotransformations to be conducted in a single liquid phase at boosted concentrations of the dissolved O-2 (here, up to 43 mM). We take reactions of glucose oxidase and d-amino acid oxidase as exemplary cases to demonstrate that the pressurized microreactor presents a powerful engineering tool uniquely apt to overcome restrictions inherent to the individual O-2-dependent transformation considered. Using soluble enzymes in liquid flow, we show reaction rate enhancement (up to six-fold) due to the effect of elevated O-2 concentrations on the oxidase kinetics. When additional catalase was used to recycle dissolved O-2 from the H2O2 released in the oxidase reaction, product formation was doubled compared to the O-2 supplied, in the absence of transfer from a gas phase. A packed-bed reactor containing oxidase and catalase coimmobilized on porous beads was implemented to demonstrate catalyst recyclability and operational stability during continuous high-pressure conversion. Product concentrations of up to 80 mM were obtained at low residence times (1-4 min). Up to 360 reactor cycles were performed at constant product release and near-theoretical utilization of the O-2 supplied. Therefore, we show that the pressurized microreactor is practical embodiment of a general reaction-engineering concept for process intensification in enzymatic conversions requiring O-2 as the cosubstrate.