X-ray-to-optical flux ratio (X/O) as a function of the hardness ratio, for the sources of Pović et al. (2009) (grey dots) in the OTELO catalogue. The horizontal line corresponds to the limit X/O = −1, which separates AGNs (X/O >  −1) from SFGs (X/O <  −1). The two vertical lines correspond to the limits set by Della Ceca et al. (2004) which encloses type 1 AGNs (−0.75 <  HR <  −0.35), represented here by blue circles. Green squares are sources with HR >  −0.35, i.e. type 2 AGNs. The pink diamond represents a probable non-AGN source, which could be a coronal emitting star, a star-forming or early-type galaxy, or a heavily absorbed (Compton thick) AGN.