X-ray-to-optical flux ratio (X/O) defined by Eq. (3), as a function of the optical band (R in Vega magnitudes) of the X-ray emitting sources in the OTELO catalogue. Red dots correspond to the sources from the original catalogue of Pović et al. (2009) with information in the soft band (0.5–2 keV), while the green diamonds are the ones from the catalogue of Laird et al. (2009). The adopted criterion to select AGNs is X/O >  −1 (dashed line). The bar represents the typical error propagated to the X/O ratio at this boundary for this sample. A total of 42 AGN counterparts was found regardless of the effects of this uncertainty estimation (see the text).