The Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) is being designed as a Day-One optical instrument for the 10.4 mts Gran Telescopio CANARIAS (GTC). It will be the first instrument, on such a large telescope, belonging to a new class of tunable spectrographs, implementing last advances in Volume Phase Holographic Gratings and tunable imaging combined with charge shuffling capabilities, covering the optical wavelength range. OSIRIS< to be first mounted dat one of GTC's Nasmyth platforms, is designed to be compact enough to fit at the Cassegrain focus as well. The optical design is devised around the classical concept of collimator plus camera. The collimator is an off axis ellipsoidal mirror, while the f/2.475 camera consists of several groups of all spherical surfaces lenses, forming a unit together with the detector rand cryocooler. A folder mirror prevents interference with the GTC acquisition and guiding subsystem. Several combinations of color and interference filters. TFs and VPHs are available in the collimated beam, near the pupil, to provide the wide versatility of required observing modes and resolutions. Short descriptions of the OSIRIS geometry, specifications, design strategy and the optical design are presented.