Gonzo, by trade I am a master decorator and so have met this issue many times in the last year I have been painting miniatures.
Generally, if a metal is coated in oil, it becomes 'freecoat', that is to say, the metal is protected from the elements and can then be coated in any paint medium.
As a precaution, I would not paint metallic acrylic onto oil, give it an acrylic varnish first, then paint.
Other than that, good to go.