  We extend a model of two-center interference to include the superposition of
opposite orientations in aligned polar molecules. We show that the position of
the minimum in the harmonic spectrum from both aligned and oriented CO depends
strongly on the relative recombination strength at different atoms, not just
the relative phase. We reinterpret the minimum in aligned CO as an interference
between opposite orientations, and obtain good agreement with numerical
calculations. Inclusion of the first-order Stark effect shifts the position of
the interference minimum in aligned CO even though aligned molecules do not
posses total permanent dipoles. We explain the shift in terms of an extra phase
that the continuum electron of oriented CO accumulates due to the Stark effect.
