OPLS is not CHARMM
ITP/GROMACS is also not CHARMM
This, however, is a CHARMM forum.
Please ask your question in a relevant forum
Scientifically: If you intend to run a simulation using the CHARMM36 force field for your protein or whatever together with a ligand parameterized to for the OPLS force field (if that is what LIGPargen does) you should be prepared to answer questions from manuscript reviewers about this strategy.