OTELO (OSIRIS Tunable Emission Line Object Survey) is the GTC Key Project that will allow an optimal exploitation of the Spanish Day One GTC instrument OSIRIS with a high scientific output and international impact. The survey will be made using tunable filters, a unique feature of OSIRIS in 8-10 m telescopes, to do co-mobile tomography at a depth enabling measurements to be made of the faintest emission line sources, while scanning a volume of the Universe containing a statistically significant sample of such objects. With this technique, and given the high sensitivity gained by the use of the tunable filters, OTELO will be the deepest and richest survey of emission line objects to date, providing an enormous amount of valuable data to tackle a wide variety of first-rank scientific projects.