  We extend the analysis of Misaligned Supersymmetry to orientifold vacua. The
lack of modular invariance in the Klein bottle, annulus and M\"obius strip
amplitudes implies that the IR properties of the physical spectrum are related
to the UV behaviour of closed strings freely propagating between D-branes and
O-planes, and \emph{vice versa}. We thus build sector averaged sums $\langle d
(n) \rangle$ associated to both representations of each Riemann surface. We
find that the $\langle d (n) \rangle$'s associated to the loop channel control
the coupling of closed-string tachyons to D-branes and/or O-planes, and
therefore their vanishing is a necessary, but not sufficient, condition for
stability in the closed sector. Similarly, the sector averaged sums associated
to the tree-level amplitudes encode the presence of tachyons in the physical
spectrum, and their vanishing is a sufficient, but not necessary, condition for
classical stability.
  Using this technology, it is difficult, if not impossible, to formulate
necessary and sufficient criteria for the absence of tachyons since, in
orientifold vacua, they can be removed from the spectrum by the orientifold
projection. Although in these cases one would naively expect a cancellation
between $\langle d (n)\rangle$'s associated to different amplitudes, this does
not occur in practice. We provide several examples in ten and lower dimensions
supporting our results.
