The structures of a semidilute polymer solution, comprised of ultrahigh molecular weight polystyrene as a solute and dioctyl phthalate as a solvent, under oscillatory shear flow were investigated by means of small-angle light scattering. The system exhibited a double-winged anisotropic scattering pattern called "butterfly", characteristic of shear-induced phase separation known to occur under a continuous shear flow. Under oscillatory shear flow the phase separation strongly depended on both a strain amplitude gamma(0) and an angular frequency omega, being observed inside a quasi-parabolic line in the space of gamma(0) vs log omega. The system was brought to a dynamically stationary state in about 20 min after applying the shear flow, and the shear-induced structures changed with a strain phase phi of the oscillatory shear. At a given phi and gamma(0), the shear-induced structures strongly depended on omega: the characteristic wavelength of the structures decreased with increasing omega, and the mean square of the concentration fluctuations was a maximum at a certain omega.