Hi there
Often an OML will be applicable to Class 1 operations only, i.e. paid commercial flying duties. If you have an OML it will often state, as mine currently does, that it does not apply to Class 2 flying which is unrestricted. I suppose this depends on the condition.
I'm not a medical expert, however I completed all of my training (MEP, CPL, MEIR) with a Class 2 medical, then only got my Class 1 (with OML) renewed shortly after my CPL LST. My FTO did not have a problem with this OML restriction, nor did the CAA examiners I flew with. Both advised that only a Class 2 was required for all training and LST flights anyway. The Class 1 is required for CPL application and licence issue.
Therefore for training purposes this should not be a problem. However, I would if I were you be certain that this is not going to be an issue when it comes to operational flying, or to know exactly how it will affect you. This is where others will be best placed to provide answers.
Hope this helps.