There is a sense in which possibility is not opposed to necessity but is, in fact, a consequence of it. All necessity, as such, entails possibility inasmuch as something necessary cannot contain any repugnance within itself. The source of possibility is, in this case, ontologically posterior to necessity.
Possibility is also unopposed to necessity inasmuch as something possible undergoes motion, meaning the reduction of some potency to act. That which was first in potency and later in act can be necessary.
Ontologically speaking, possibility and necessity are opposed to one another when possibility is taken to mean contingent. Contingency is the possibility for non-existence, and that which is potentially non-existent cannot possibly be necessary.