Emission-line galaxies are important targets for understanding the chemical evolution of galaxies in the universe. Deep, narrowband imaging surveys allow detection and study of the flux and the equivalent widths (EWs) of the emission line studied. The present work has been developed within the context of the OTELO project, an emission-line survey using the tunable filters (TF) of OSIRIS, the first-generation instrument on the Gran Telescopio Canarias (GTC) 10.4 m telescope located in La Palma, Spain, that will observe through selected atmospheric windows that are relatively free of sky emission lines. With a total survey area of 0.1 deg2 distributed in different fields, reaching a 5σ depth of 10-18 ergs cm-2s-1 and detecting objects of EW < 0.3 Å , OTELO will be the deepest emission-line survey to date. As part of the OTELO preparatory activities, the objective of this study is to determine the best combination of sampling and full width at half-maximum (FWHM) for the OSIRIS tunable filters for deblending Hα from [N II] lines by analyzing the flux errors obtained. We simulated the OTELO data by convolving a complete set of synthetic H II galaxies in EWs, with different widths of the OSIRIS TFs. We estimated relative flux errors of the recovered Hα and [N II] λ6583 lines. We found that for the red TF, a FWHM of 12 Å and a sampling of 5 Å is an optimal combination that allows deblending Hα from the [N II] λ6583 line with a flux error lower than 20%. This combination will allow estimating SFRs and metallicities using the Hα flux and the N2 method, respectively.