Tight stability requirements for the imager/spectrograph OSIRIS (a Day One optical instrument for the GTC telescope) demand a careful treatment of thermal effects within the OSIRIS camera. Mostly due to the thermal response of refraction indices of its glasses (and not so much to curvature, spacing or thickness variations of the lenses), the camera optics alone degrades beyond requirements the image quality and plate scale under the expected ambient temperature variations (about 1.8 °C/hour). Thermal effects and thermal compensator studies of the OSIRIS camera are first summarized, before discussing how the motion (of a few microns per °C) of the 3rd camera doublet, as a sole compensator, practically eliminates thermal influences on both image quality and plate scale. A concept for the passive implementation of the compensator is also discussed.