Context : The OSIRIS Tunable Filter Emission Line Object ( OTELO ) survey is a very deep , blind exploration of a selected region of the Extended Groth Strip and is designed for finding emission-line sources ( ELSs ) .
The survey design , observations , data reduction , astrometry , and photometry , as well as the correlation with ancillary data used to obtain a final catalogue , including photo-z estimates and a preliminary selection of ELS , were described in a previous contribution .

Aims : Here , we aim to determine the main properties and luminosity function ( LF ) of the [ O III ] ELS sample of OTELO as a scientific demonstration of its capabilities , advantages , and complementarity with respect to other surveys .

Methods : The selection and analysis procedures of ELS candidates obtained using tunable filter ( TF ) pseudo-spectra are described .
We performed simulations in the parameter space of the survey to obtain emission-line detection probabilities .
Relevant characteristics of [ O III ] emitters and the LF ( [ O III ] ) , including the main selection biases and uncertainties , are presented .

Results : From 541 preliminary emission-line source candidates selected around z=0.8 , a total of 184 sources were confirmed as [ O III ] emitters .
Consistent with simulations , the minimum detectable line flux and equivalent width ( EW ) in this ELS sample are \sim 5 \times 10 ^ { -19 } erg s ^ { -1 } cm ^ { 2 } and \sim 6 Å , respectively .
We are able to constrain the faint-end slope ( \alpha = -1.03 \pm 0.08 ) of the observed LF ( [ O III ] ) at a mean redshift of z=0.83 .
This LF reaches values that are approximately ten times lower than those from other surveys .
The vast majority ( 84 % ) of the morphologically classified [ O III ] ELSs are disc-like sources , and 87 % of this sample is comprised of galaxies with stellar masses of M _ { \star } < 10 ^ { 10 } M _ { \odot } .

Conclusions :