  There is a longstanding puzzle concerned with the existence of Op~-planes
with p>=6, which are orientifold p-planes of negative charge with stuck
Dp-branes. We study the consistency of configurations with various orientifold
planes and propose a resolution to this puzzle. It is argued that O6~-planes
are possible in massive IIA theory with odd cosmological constant, while
O7~-planes and O8~-planes are not allowed. Various relations between
orientifold planes and non-BPS D-branes are also addressed.
