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

Tachyons and Misaligned Supersymmetry in Orientifold Vacua