We report on, to the best of our knowledge, the first singly resonant (SR), synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned gallium arsenide (OP-GaAs). Together with a doubly resonant (DR) degenerate OPO based on the same OP-GaAs material, the output spectra cover 3 to 6 μm within ∼3 dB of relative power. The DR-OPO has the highest output power reported to date from a femtosecond, synchronously pumped OPO based on OP-GaAs. We observed strong three-photon absorption with a coefficient of 0.35±0.08 cm3/GW2 for our OP-GaAs sample, which limits the output power of these OPOs as mid-IR light sources. We present a detailed study of the three-photon loss on the performance of both the SR- and DR-OPOs, and compare them to those without this loss mechanism.