  We report on the discovery of X-ray--emitting O-Ne-Mg-rich ejecta in the
middle-aged Galactic O-rich supernova remnant Puppis A with Chandra and
XMM-Newton. We use line ratios to identify a low-ionization filament running
parallel to the northeastern edge of the remnant that requires supersolar
abundances, particularly for O, Ne, and Mg, which we interpret to be from
O-Ne-Mg-rich ejecta. Abundance ratios of Ne/O, Mg/O, and Fe/O are measured to
be ~2, ~2, and <0.3 times the solar values. Our spatially-resolved spectral
analysis from the northeastern rim to the western rim otherwise reveals
sub-solar abundances consistent with those in the interstellar medium. The
filament is coincident with several optically emitting O-rich knots with high
velocities. If these are physically related, the filament would be a peculiar
fragment of ejecta. On the other hand, the morphology of the filament suggests
that it may trace ejecta heated by a shock reflected strongly off the dense
ambient clouds near the northeastern rim.
