  We classify all equivalences between the indecomposable abelian categories
which appear as blocks in BGG category O for reductive Lie algebras. Our
classification implies that a block in category O only depends on the Bruhat
order of the relevant parabolic quotient of the Weyl group. As part of the
proof, we observe that any finite dimensional algebra with simple preserving
duality admits at most one quasi-hereditary structure.
