PISN 2018ibb: radioactive emission of [O III] lines

Supernova 2018ibb of the PISN category related to the dynamical instability of oxygen core in a supermassive star induced by pair-creation shows at the nebular stage strong [\oiii] emission lines of an uncertain origin. I propose a simple model that demonstrates a possibility of [O III] lines emission from the supernova oxygen matter ionized and heated by the $^{56}$Co radioactive decay. The reason is pinpointed by which the [O III] line luminosity among supernovae of PISN category can vary in a broad range.