I agree that an explanation is far more valuable than an oml, but an oml is still better than no answer at all(maybe not for future readers, but for the original poster), so I would prefer it to be a gentle nudge rather than a block from replying.

How are you thinking of enforce this ? Maybe AI analysing the answer before it's posted, with a suggestion to add some more information ?

Yes, something like that. It must be clear from the text what the answer is, and if an OML with a more detailed solution is attached, that is fine. But it should not be the only thing. We don't need a fancy AI thingy. Just scanning on the sentence count en if the message is nog equal "Here is the  OML" ;-)

It is all about learning from the forum post; OML for more detail if needed

Fully agree that an explanation of the change is valuable. I don't think we can or should enforce this. As moderators or any community member, we can educate ourselves and others to share thoughts about a discussion. In the past we were also able to influence how people participate on the forum.