The conceptual design of the GTC optical system was completed in summer of 1997. It will be a Richey Cretien type telescope with a flat tertiary mirror to feed Nasmyth and folded Cassegrain focal stations. The telescope will have a segmented primary mirror and an entrance pupil area equivalent to a circular aperture of 10 m diameter. The details of its optical- and opto-mechanical design have been chosen to meet the scientific requirements such as excellent image quality in a broad wavelength range (0.3 microns < (lambda) < 15 microns) and maximum operational efficiency. To achieve the required specifications, all optical elements, in particular the primary mirror segments need to be equipped with a high performance opto-mechanical support system. In this paper we present the results of the optical- and opto-mechanical design of the GTC.