We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to $10.7\mu \mathrm{m}$ , measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OP-GaAs sample with a $166.6\text{-}\mu \mathrm{m}$ quasi-phase-matching period was pumped with $3.1\text{–}3.3\mu \mathrm{m}$ wavelength, 1 ps pulses up to $2\mu \mathrm{J}$ in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%.