Oscillating wings can extract energy from an oncoming water or air stream, and first large-scale marine demonstrators are being tested. Oscillating wing hydrodynamics is highly unsteady, may feature dynamic stall and leading edge vortex shedding, and is significantly three-dimensional due to finite-wing effects. Understanding the interaction of these phenomena is essential for maximizing power generation efficiency. Much of the knowledge on oscillating wing hydrodynamics stemmed from two-dimensional low-Reynolds number computational fluid dynamics studies and laboratory testing; real installations, however, will feature Reynolds numbers higher than 1 million and unavoidable finite-wing-induced losses. This study investigates the impact of flow three-dimensionality on the hydrodynamics and the efficiency of a realistic aspect ratio 10 device in a stream with Reynolds number of 1.5 million. The improvements achievable by using endplates to reduce finite-wing-induced losses are also analyzed. Three-dimensional time-dependent Navier–Stokes simulations using the shear stress transport turbulence model and a 30-million-cell grid are performed. Detailed comparative hydrodynamic analyses of the finite and the infinite wings reveal that flow three-dimensionality reduces the power generation efficiency of the finite wing with sharp tips and that with endplates by about 17% and 12% respectively. Presented analyses suggest approaches to further reducing these power losses.