OSIRIS (Optical System for Imaging and low/intermediate-Resolution Integrated Spectroscopy) is an instrument designed to obtain images and low resolution spectra of astronomical objects in the optical domain (from 365 through 1000nm). It will be installed on Day One in the Nasmyth focus of the 10-meter Spanish GTC Telescope, although it shall be possible to install it in the Cassegrain focus as well. It is expected to be in operation at the end of 2003. The Slit Unit is an automated slit mask loader based in cam followers technology, being designed at the IAC for the OSIRIS Spectrograph. It provides a store with space for as many as 13 multislits mask and long slit mask for an unvignetted field of view of 8.53'x8.67', available at the same time for observing purposes. A two-degrees-of-freedom mechanism allows to select one of the masks, to remove it from the cassette and to position it in the focal plane with the required repeatability. The complete design of the mechanism is presented, including an analysis of the predicted performances and a 3-D model used to check the geometry and mass properties.