Terahertz wave (THz) photoconductive (PC) antennas were fabricated on oxygen-implanted GaAs (GaAs:O) and low-temperature-grown GaAs (LT-GaAs). The measured cw THz power at $0.358\mathrm{THz}$ from the GaAs:O antenna is about twice that from the LT-GaAs antenna under the same testing conditions, with the former showing no saturation up to a bias of $40\mathrm{kV}∕\mathrm{cm}$ , while the latter is already beginning to saturate at $20\mathrm{kV}∕\mathrm{cm}$ . A modified theoretical model incorporating bias-field-dependent electron saturation velocity is employed to explain the results. It shows that GaAs:O exhibits a higher electron saturation velocity, which may be further exploited to generate even larger THz powers by reducing the ion dosage and optimizing the annealing process in GaAs:O.