  We present results of the first investigations on the correlated nature of
electronic states that cross the Fermi level in Pb$_9$Cu(PO$_4$)$_6$O aka LK-99
obtained within the DFT + DMFT approach. Coulomb correlations between Cu-$d$
electrons led to the opening of the band gap between the extra-O $p$ and Cu
$d_{xz}/d_{yz}$ states. We state that oxygen $p$ states play a significant role
in the electronic properties of LK-99. We also assume that doping with
electrons is necessary to turn the stoichiometric Pb$_9$Cu(PO$_4$)$_6$O into
conducting state.
