We present the current design of WFOS, a wide-field UV/optical (0.31-1.0 mu m) imaging spectrograph planned for first-light on the TMT International Observatory 30 m telescope. WFOS is optimized for high sensitivity across the entire optical waveband for low-to-moderate resolution (R similar to 1500-5000) long-slit and multi-slit spectroscopy of very faint targets over a contiguous field of view of 8'.3x3'.0 at the f/15 Nasmyth focus of TMT. A key design goal for WFOS is stability and repeatability in all observing modes, made possible by its gravity-invariant opto-mechanical structure, with a vertical rotation axis and all reconfigurable components moving only in planes defined by tiered optical benches parallel to the Nasmyth platform. WFOS's optics include a linear ADC correcting a 9' diameter field, including both the science FoV and 4 patrolling acquisition, guiding, and wavefront sensing camera systems; a novel 2-mirror reflective collimator allowing the science FoV to be centered on the telescope optical axis; a dichroic beamsplitter dividing the collimated beam into 2 wavelength-optimized spectrometer channels (blue: 0.31-0.56 mu m; red: 0.54-1.04 mu m); selectable transmissive dispersers (VPH and/or VBG) with remotely configurable grating tilt (angle of incidence) and camera articulation that enable optimization of diffraction efficiency and wavelength coverage in each channel; all-refractive, wavelength-optimized f/2 spectrograph cameras, and UV/blue and red-optimized detector systems. The predicted instrumental throughput of WFOS for spectroscopy averages > 56% over the full 0.31-1 mu m range, from the ADC to the detector. When combined with the 30 m TMT aperture, WFOS will realize a factor of similar to 20 gain in sensitivity compared to the current state of the art on 8-10 m-class telescopes.