  The observational characteristics of quasi-periodic oscillations (QPOs) from
accreting neutron stars strongly indicate the oscillatory modes in the
innermost regions of accretion disks as a likely source of the QPOs. The inner
regions of accretion disks around neutron stars can harbor very high frequency
modes related to the radial epicyclic frequency $\kappa $. The degeneracy of
$\kappa $ with the orbital frequency $\Omega $ is removed in a non-Keplerian
boundary or transition zone near the magnetopause between the disk and the
compact object. We show, by analyzing the global hydrodynamic modes of long
wavelength in the boundary layers of viscous accretion disks, that the fastest
growing mode frequencies are associated with frequency bands around $\kappa $
and $\kappa \pm \Omega $. The maximum growth rates are achieved near the radius
where the orbital frequency $\Omega $ is maximum. The global hydrodynamic
parameters such as the surface density profile and the radial drift velocity
determine which modes of free oscillations will grow at a given particular
radius in the boundary layer. In accordance with the peak separation between
kHz QPOs observed in neutron-star sources, the difference frequency between two
consecutive bands of the fastest growing modes is always related to the spin
frequency of the neutron star. This is a natural outcome of the boundary
condition imposed by the rotating magnetosphere on the boundary region of the
inner disk.
