Illustrative example of an extreme [OIII] emitter, the J-PLUS source 67834-5013 (RA = 239.1019, Dec = 48.1127, z_spec = 0.050). Left panel: color composite of the galaxy, obtained from the gri J-PLUS images. The sky location of the source is shown as a black dot, and the white ellipse indicates the three effective radius contours for the source. Right panel: J-PLUS 12-band PSFCOR photometry of the galaxy. The squares show the five SDSS-like filters (ugriz), and circles indicate the seven medium-band filters (J0378, J0395, J0410, J0430, J0515, J0660, and J0861). The solid line shows the spectra from SDSS with a downgraded resolution of R ∼ 180, normalized to the flux in the filter J0660. The location of the most prominent emission lines is marked: [OIII] (traced by J0515 and g), Hα (traced by r), and [OII] (traced by J0395).