We perform a sequence of 3D magnetohydrodynamic (MHD) simulations of the outflow-core interaction for a massive protostar forming via collapse of an initial cloud core of 60 M☉. This allows us to characterize the properties of disk-wind-driven outflows from massive protostars, which can allow testing of different massive star formation theories. It also enables us to assess quantitatively the impact of outflow feedback on protostellar core morphology and overall star formation efficiency (SFE). We find that the opening angle of the flow increases with increasing protostellar mass, in agreement with a simple semianalytic model. Once the protostar reaches ∼24 M☉, the outflow's opening angle is so wide that it has blown away most of the envelope, thereby nearly ending its own accretion. We thus find an overall SFE of ∼50%, similar to that expected from low-mass protostellar cores. Our simulation results therefore indicate that the MHD disk wind outflow is the dominant feedback mechanism for helping to shape the stellar initial mass function from a given prestellar core mass function.