  The amplification of disk oscillations resulting from non-linear resonant
couplings between the oscillations and a warp is examined. The disks are
geometrically thin and general relativistic with non-rotating central objects.
By using a Lagrangian formulation, a general stability criterion is derived.
The criterion is applied to horizontal and vertical resonances of g-mode
oscillations and to horizontal resonances of p-mode oscillations. The results
of analyses show that g-mode oscillations (including p-mode oscillations of
$n=0$) are amplified by horizontal resonances with the warp. Other modes of
oscillations with other resonances are all damped by resonances. The amplified
g-mode oscillations are located around the radius $4r_{\rm g}$, the radius of
the epicyclic frequency being maximum, i.e., $\kappa_{\rm max}$. The
frequencies of amplified oscillations are harmonics of $\kappa_{\rm max}$, and
well explain the 2 : 3 pairs of observed QPOs for reasonable masses (and spins)
of the central objects.
