I need melted butter
I would say no for two main reasons, the butter will change texture as the dessert cools, and I don't think the oil will, so the mousse will split. Also the tastes will be off.

You could try margarine, or at a real push lard (you would need to play around with the sugars to keep the sweetness)

However I have seen recipies that exclude butter,  here is a link to one possibility.

http://www.sarahcooks.com.au/2012/07/perfect-chocolate-mousse.html 

Good luck