IEEE Reviewer Comment Abbreviations
I've never seen those abbreviations in a review or in any of the IEEE guidelines I looked at. I suspect those are either specific to a certain journal or—more probably—of the reviewer's personal style. I'd guess that the reviewer wanted to grade their remarks and requests of amendments and the meaning could be as follows:

MA(JOR)
MI(NOR)
OP(TIONAL)

If the above doesn't fit and you think those labels may influence your revision in a significant way, contact the editor for clarification.