The night before - where should my Mog stay?

I would ask her. It sounds as if your OH would like her to be at home with him, and that may be her preference too.

Sounds like your H2B really wants his Mum to be with him. Although with weddings their is no "done thing" really, I would say that it is not very often that the MOG is with the bride and maids/ MOB getting ready.

In cases when everything is at a venue and they are all staying the night before - both set s of parents I do see the MOG maybe have some hair/makeup done but not often.

It doesn't sound like you have a problem compared to the other way around -- if his Mum was expecting to be with you and you were not planning on that happening

I would suggest that your fiance tell his mother how much he would like her to be there for him on the morning of his weding, and that you are happy for her to drop out of your group so that she can be there. I can't imagine any mother not wanting to be with her son on his wedding morning if she knows that is what he wants, but she may feel awkward about cancelling her agreement to get ready with you unless she is told that it's not a problem.