It surely depends upon what the OML is for; only the CAA can really give you an answer to the question. I have seen an absurd situation where a pilot has an OML on a Class 1 but not on a Class 2. He can give ab initio instruction on the Class 2, if unremunerated.
If you were not allowed to teach prior to 1st solo, there would be little point being an FI! in any event, it would have no bearing on 25 solo signs off as they are all post first solo!