We have used room temperature photoreflectance spectroscopy to study interfacial electronic properties of various oxide-GaAs heterostructures. The samples, air-, Al2O-, Ga2Ox-, and Ga2O3(Gd2O3)-GaAs, were fabricated by in-situ molecular beam epitaxy. Built-in electric fields are 48, 44, and 35 kV/cm for air-, Al2O3-, and Ga2Ox-GaAs samples, respectively, corresponding to the interfacial state density (Dit) of 2.4, 2.2, and 1.9×1011 cm-2 eV-1, respectively. For the Ga3O3(Gd2O3)-GaAs sample, the built-in electric field is negligibly small, indicating a very low interfacial stare density. Estimated by the low field limit criterion, Dit is less than 1×1011 cm-2 eV-1. Our results are consistent with the previous data obtained using capacitance-voltage measurements in quasi-static/high frequency modes.