[Oi]-based diagnostic diagrams for the optical-radio sample. From top to bottom and from left to right, log (L_{20 cm}/L_Hα) increases. The number of radio emitters per bin is constant and equal to 2350 ± 25. The contours represent the number density of the radio emitters (20 galaxies per density contour). The bulk of the population shifts from the SF region to the composite or AGN region with increasing L_{20 cm}/L_Hα. The transition seems to occur for log (L_{20 cm}/L_Hα) > 0.716.