  We obtain an M--theory interpretation of different IIA orientifold planes by
compactifying them on a circle and use a chain of dualities to get a new IIA
limit of these objects using this circle as the eleventh dimension. Using the
combination of the two IIA description, we give an interpretation for all
orientifold four-planes in M-theory, including a mechanism for freezing
M5-branes at singularities.
