Radio luminosity (L_r) versus [OIII] luminosity for the different radio-optical classes. Triangles correspond to the B10 sample, and circles the 3CR-VLBI subsample. The 3CR radio luminosities are measured at 178 MHz, whereas the 3CR-VLBI luminosities are at 8 GHz. The 178 MHz luminosities have been shifted by one decade (+1) to clarify the comparison between low and high frequencies. The vertical dashed-dotted green lines indicate the lower and upper [OIII] luminosity values for the 3CR FRII-LERGs, highlighting how the separation of RGs into different radio-optical classes is notably similar at 178 MHz and 8 GHz.