OPESumSimplify[exp] simplifies OPESum's in exp.
See also: OPESum, OPESumExplicit.
OPESum[(-SOD[p])^(OPEi+1) SOD[p-q]^(OPEm-OPEi-2),{OPEi,0,OPEm}]
OPESumSimplify[%]