  We determine the oxygen density in the central zone of nine type IIP
supernovae (SN~IIP) at the nebular stage using oxygen doublet [O I] 6300, 6364
\AA. Combined with two available estimates these data indicate that oxygen
densities on day 300 are distributed in rather narrow range
$(2.3\pm1)\times10^9$ cm$^{-3}$. The result does not depend on the distance,
extinction, or model assumptions. We demonstrate that the found density
distribution suggests that the explosion energy of SN~IIP increases with the
stellar mass.
