OPESumSimplify[exp] simplifies OPESums in exp.
Overview, OPESum, OPESumExplicit.
OPESum[(-SOD[p])^(OPEi + 1) SOD[p - q]^(OPEm - OPEi - 2), {OPEi, 0, OPEm}]\sum _{i=0}^m (-(\Delta \cdot p))^{1+i} (\Delta \cdot (p-q))^{-2-i+m}
OPESumSimplify[%](\Delta \cdot p) \left(-\sum _{i=0}^m (-1)^i (\Delta \cdot p)^i (\Delta \cdot (p-q))^{-2-i+m}\right)
OPESumSimplify[OPESum[{OPEi, 0, OPEm}] a^OPEi]\sum _{i=0}^m a^i
OPESumSimplify[OPESum[{j, 0, i}, {i, 0, m}] a^(j - i) b^i]\sum _{i=0}^m \;\text{}\;\text{} (i+1)b^i a^{j-i}
% // StandardForm
(*OPESum[a^(-i + j) b^i, {i, 0, m}, {j, 0, i}]*)