WYFFOS is a spectrograph, currently being assembled, for multi- object spectroscopy using fiber-optic inputs. Initially 126 fibers, later 150 finer fibers (described in a separate paper) will enter it from the prime focus of the 4.2-meter William Herschel Telescope. WYFFOS uses the Baranne white-pupil, Littrow- mount optical system, given a wide 2D filed of view at the grating, high efficiency and a wide range of spectral resolutions. It has a twice-through lenticular collimator and a Schmidt-type cryogenic camera, operating from 350 nm to one micron. We shall install the spectrograph on an optical bench on one of the Nasmyth platforms of the telescope. One advantage of using the optical bench is that the spectrograph's components can be re-arranged, converting the reflection-grating instrument to a transmission-grating instrument. Possible future enhancements include increasing the number of fibers to 300 in the transmission grating mode, and adding a dioptric camera.