  The Oort constants describe the local variations of the stellar streaming
field. Classically, they are determined from stellar proper motions. We discuss
problems arising in this procedure. A large, hitherto overlooked, source of
systematic error arises from longitudinal variations of the mean stellar
parallax. Together with the solar reflex motion, these variations create
contributions to the longitudinal proper motions mu_l*(l) that are
indistinguishable from the Oort Constants at the 20% level. Fortunately, we can
correct for this "mode mixing" using the latitudinal proper motions mu_b(l). We
use ~10^6 stars from the ACT/Tycho-2 catalogs with proper motion errors of ~ 3
mas/yr. We find significant deviations from expectations based on a smooth
axisymmetric equilibrium disk, in particular non-zero C for old red giant
stars. We also find variations of the Oort constants with the asymmetric drift
of the sub-sample. These correlations are different in nature than those
expected for an axisymmetric Galaxy. The most reliable tracers for the ``true''
Oort constants are red giants, which are old enough to be in equilibrium and
distant enough to be unaffected by possible local anomalies. For these stars we
find, A ~ 16, B ~- 17, A - B ~ 33, and C ~- 10 km/s/kpc with internal errors of
about 1-2 and external error of perhaps the same order. These values are
consistent with our knowledge of the Milky Way (flat rotation curve and Omega=A
- B ~ 28 +/- 2). Based on observations made with the ESA Hipparcos astrometry
satellite. (Abridged)
