  Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of
three parameters, i.e., the white dwarf mass, the envelope mass at ignition,
and the initial composition. A quasi-analytic one-zone approach is used with an
up-to-date nuclear reaction network. The nucleosynthesis results show
correlation with the peak temperatures or the cooling timescales during
outbursts. Among the combinations of white dwarf and envelope masses which give
the same peak temperature, the explosion is more violent for a lower white
dwarf mass owing to its smaller gravitational potential. Comparison of the
nucleosynthesis results with observations implies that at least two-third of
the white dwarf masses for the observed ONeMg novae are $\simeq 1.1 M_\odot$,
which are significantly lower than estimated by previous hydrodynamic studies
but consistent with the observations of V1974 Cyg. Moreover, the envelope
masses derived from the comparison are $\gtrsim 10^{-4} M_\odot$, which are in
good agreement with the ejecta masses estimated from observations but
significantly higher than in previous hydrodynamic studies. With such a low
mass white dwarf and a high mass envelope, the nova can produce interesting
amounts of $\gamma$-ray emitters $^7$Be, $^{22}$Na, and $^{26}$Al. We suggest
that V1974 Cyg has produced $^{22}$Na as high as the upper limit derived from
the COMPTEL survey. In addition, a non-negligible part of the Galactic
$^{26}$Al may originate from ONeMg novae, if not the major contributors. Both
the future INTEGRAL survey for these $\gamma$-ray emitters and abundance
estimates derived from ultraviolet, optical, and near infrared spectroscopies
will impose a severe constraint on the current nova models.
