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 (Cepa et al. 2008). Observing in selected atmospheric windows relatively free of sky emission lines, and with a total survey area of 0.1square degree distributed in different fields, it is expected to reach 5 sigma depth of 10-18erg/cm2/s, detecting objects with 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 Hα from [Nii]λ6,583 lines with a flux error lower than 20%. With the selected instrumental configuration it 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 3,000 Hα star forming emitters up to a redshift 0.4.