  We compare the theoretical predictions of the transition layer model with
some observational features of quasi-periodic oscillations (QPOs) in neutron
star X-ray binaries. We found that the correlation between horizontal branch
oscillation (HBO) frequencies and kilohertz (kHz) QPO frequencies, the
difference between the low-frequency QPOs in atoll sources and HBOs in Z
sources, and the correlation between the frequencies of low-frequency QPOs and
break frequencies can be well explained by the transition layer model, provided
the neutron star mass is around 1.4 solar mass and the angle between
magnetosphere equator and accretion disk plane is around 6 degree. The observed
decrease of peak separation between two kHz QPO frequencies with the increase
of kHz QPO frequencies and the increase of QPO frequencies with the increase of
inferred mass accretion rate are also consistent with the theoretical
predictions of transition layer model. In addition, we derive a simple equation
that can be adopted to estimate the angle ($\delta$) between magnetosphere
equator and accretion disk plane by use of the simultaneously observed QPO
frequency data. We estimate these angles, in the range of 4 to 8 degrees, for
five Z sources and two atoll sources. The nearly constant $\delta$ value for
each source, derived from the different sets of simultaneously observed QPO
frequency data, provides a strong test of the theoretical model. Finally, we
suggest that the similar transition layer oscillations may be also responsible
for the observed QPOs in accretion-powered millisecond X-ray pulsar and
Galactic black hole candidates.
