The use of oscillating foils to develop turbines to extract energy from an oncoming flow has recently been found competitive with rotor blade designs. Moreover, the rectangular extraction plane of the oscillating-foil turbine presents clear advantages in the field of hydrokinetic turbines. The present study, based on two-dimensional unsteady Reynolds-averaged Navier–Stokes numerical simulations, focuses on the range of operating parameters maximizing power extracted by a single oscillating foil for small-scale application Reynolds number. The power-extraction efficiency is mapped in parametric spaces associated with different motion amplitudes. Efficiency maxima reach about 43% in all the parametric spaces considered, but for different values of pitching amplitude and frequency. It is found that the best parametric zones in all cases exhibit similar characteristics with regards to the effective angle of attack. Indeed, they are all associated with a maximum effective angle of attack of about 33 deg and a maximum (normalized) rate of change of the latter of 0.55. It is further observed that, contrary to laminar results previously published, the occurrence of leading-edge vortex shedding is not necessarily observed in optimal cases at these higher Reynolds number conditions.