Is it ok to cuddle with a friend of the opposite sex if you are in a relationship with someone else?

I think so, but only if there is nothing sexual going on, like groping or kissing.

That is entirely something you should be discussing with your Significant Other, and depends on the terms of the relationship. por all means, you do you, and don't let anyone dictate who you are friends with, but do keep in mind the feelings of your S.O. and be courteous and mature about your habits. If s/he is uncomfortable, allow them to express that and don't get defensive about it. Just listen. If s/he has no problem with it, then at least you were up front about it and now there should be no confusion.