I assume the question is only about "opposé(e)" in a spatial sense and not in abstract sense (like opposed to an idea).

In this case: If you want express that something is opposite of something else, as far as I can tell, you will nearly always use "en face de" and not "opposée à".

So for example, "La maison en face de la nôtre" and not "La maison opposée à la nôtre" would be used to express "the house opposite ours". "C'est juste en face." for "that's just across the street" and not "C'est juste opposé".

However, when comparing the adjective "d'en face" and "opposée" in case of the opposite river bank, it seems "opposée" is more common than "d'en face":

"la rive opposée" vs "la rive d'en face"

 

The "opposite side" e.g. of a triangle would also be: "côté opposée"

Also, you cannot use "en face" when you want the say "in the opposite direction": 

"dans la direction opposée"