SO[q] is a four-dimensional scalar product of
OPEDelta with q. It is transformed into
Pair[Momentum[q], Momentum[OPEDelta] by
FCI.
Overview, FCI, OPEDelta, Pair, ScalarProduct, SOD.
SO[p]\Delta \cdot p
SO[p - q]\Delta \cdot (p-q)
SO[p] // FCI // StandardForm
(*Pair[Momentum[OPEDelta], Momentum[p]]*)