  We perform detailed calculations of cosmic microwave background (CMB)
anisotropies in a CDM-dominated open universe with primordial adiabatic density
perturbations for a variety of reionization histories. We show that to a great
extent, the CMB anisotropies depend only on the geometry of the Universe, which
in a matter dominated universe is determined by $\Omega$, and the optical depth
to the surface of last scattering. In particular, the location of the primary
Doppler peak depends primarily on $\Omega$ and is fairly insensitive to the
other unknown parameters, such as $\Omega_b$, $h$, $\Lambda$, and the shape of
the power spectrum. Therefore, measurements of CMB anisotropies on small scales
may be used to determine $\Omega$.
