T-cut has a bad name because its probably the most aggressive polish you can get. The reason for that is because its designed for hand use and therefore needs to be aggressive to do the work.
Basically, if T-cut doesn't make an impact, I doubt anything will.
I personally would have reached for Autoglym Paint Renovator in this situation - a no nonsense product.
If it is true oxidation, it will come off easy, if not, you might require a respray. I would hazard a guess that the panel has been resprayed poorly in the past, especially since only 1 is affected. It shouldn't really be oxidising anyway as its a clearcoat-ed paint