OPESum[exp, {i, 0, m}] denotes a symbolic sum. The syntax is the same as for Sum.
Overview, OPESumExplicit, OPESumSimplify.
OPESum[SO[p]^OPEiSO[k]^(OPEm - OPEi - 3), {OPEi, 0, OPEm - 3}]
OPESumExplicit[%]\sum _{i=0}^{-3+m} (\Delta \cdot p)^{\text{OPEiSO}(k)^{-3-i+m}}
\sum _{i=0}^{-3+m} (\Delta \cdot p)^{\text{OPEiSO}(k)^{-3-i+m}}
OPESum[a^ib^(j - i) c^(m - j - 4), {i, 0, j}, {j, 0, m - 4}]
OPESumExplicit[%]\sum _{j=0}^{-4+m} \;\text{}\;\text{} (j+1)c^{-j+m-4} a^{\text{ib}^{j-i}}
\sum _{j=0}^{-4+m} \;\text{}\;\text{} (j+1)c^{-j+m-4} a^{\text{ib}^{j-i}}