We have successfully fabricated depletion mode GaAs metal-oxide-semiconductor field effect transistors using Ga2O3(Gd2O3) as the gate oxide and an oxygen implant isolation technique. Growth of the device structure including the deposition of Ga2O3(Gd2O3) was performed in a multichamber molecular beam epitaxy system. A 1 mu m x 100 mu m device shows excellent de and microwave characteristics with low output conductance. Complete pinchoff at V-g = -2.5 V and operation in the accumulation mode of up to V-g = 2.5V were measured. The maximum transconductance was 100 mS/mm, with a high drain current density of 315 mA/mm. Microwave testing yielded a f(T) of 14 GHz and a f(max) of 35 GHz.