PowerSimplify[exp] simplifies (-x)^a to
(-1)^a x^a and (y-x)^n to
(-1)^n (x-y)^n thus assuming that the exponent is an
integer (even if it is symbolic).
Furthermore, (-1)^(a+n) and I^(a+n) are
expanded and
(I)^(2 m) -> (-1)^m and (-1)^(n_Integer?EvenQ m) -> 1
and (-1)^(n_Integer?OddQ m) -> (-1)^m for n
even and odd respectively and (-1)^(-n) -> (-1)^n and
Exp[I m Pi] -> (-1)^m.
PowerSimplify[(-1)^(2 OPEm)]1
PowerSimplify[(-1)^(OPEm + 2)](-1)^m
PowerSimplify[(-1)^(OPEm - 2)](-1)^m
PowerSimplify[I^(2 OPEm)](-1)^m