  We present nebular phase optical and near-infrared spectroscopy of the Type
IIP supernova SN 2012aw combined with NLTE radiative transfer calculations
applied to ejecta from stellar evolution/explosion models. Our spectral
synthesis models generally show good agreement with the ejecta from a MZAMS =
15 Msun progenitor star. The emission lines of oxygen, sodium, and magnesium
are all consistent with the nucleosynthesis in a progenitor in the 14 - 18 Msun
range. We also demonstrate how the evolution of the oxygen cooling lines of [O
I] 5577 A, [O I] 6300 A, and [O I] 6364 A can be used to constrain the mass of
oxygen in the non-molecularly cooled ashes to < 1 Msun, independent of the
mixing in the ejecta. This constraint implies that any progenitor model of
initial mass greater than 20 Msun would be difficult to reconcile with the
observed line strengths. A stellar progenitor of around MZAMS = 15 Msun can
consistently explain the directly measured luminosity of the progenitor star,
the observed nebular spectra, and the inferred pre-supernova mass-loss rate. We
conclude that there is still no convincing example of a Type IIP explosion
showing the nucleosynthesis expected from a MZAMS > 20 Msun progenitor.
