The OTELO (OSIRIS Tunable Emission Line Object Survey project) is an emission line survey using the OSIRIS Tunable Filters on the GTC 10.4m telescope. Observing in selected atmospheric windows relatively free of sky emission lines, and with a total survey area of 0.1  square degree distributed in different fields, it is expected to reach  5 sigma depth of 10-18 erg/cm2/s, detecting objects of EW<0.2. OTELO  will be the deepest emission line survey to date. As part of the preparatory activities, we have selected from  the simulation of OTELO spectra, including  errors, the best combination of tunable filters bandwidth (FWHM) and sampling, that will allow deblending Ha from  [NII]λ6584 lines with a flux error lower than 20%. With the selected instrumental configuration will be possible to estimate the objects chemical abundances using the N2 method in very low metallicity systems. We estimate that OTELO will allow to estimate the  metallicities of more than 3000 Ha star forming emitters up to a redshift 0.4.