Symbols are usually derived from either the mathematical symbolism of the operation they perform or from the physical shape of the common implementation of mechanism.
For an OMT, there isn't really a graphical description of the mathematical operation and the physical shape of an OMT is really just a bunch of rectangles...
I think in this case it makes sense to fall back to a Box with a reference to port functions, i.e. A square bounding box with an O, X, or large + to represent the dual-pol port. The O would be for circular waveguide (but sometimes people will use a square waveguide), or perhaps use an X for cross-polarized or use a large plus for a physical representation of horizontal and vertical polarization superimposed together)
The polarized ports can simply be described by a V or an H to clearly indicate the two single-pol ports. (Or L and R if you're doing circular polarization...)