I would say, if it is important to you, then you should talk about. However, you don't have to ask. Let's assume that you two are having a dinner together at some point. You could say something to the effect of, "I do enjoy spending time with you. But it is important to me that we be exclusive with each other. You don't have to tell me anything about your relationship with OM, but if we are not being exclusive together, then I would rather we didn't spend time together." That lets her know what your boundary is. And she knows how to act based on her situation, which she doesn't even have to confide to you.
