In music:
* An overture is an instrumental piece introducing an opera, ballet, or other dramatic work. There isn't a specific term for an ending piece, though "finale" or "closing piece" could be used.
In general:
* An overture is an introductory act, proposal, or offer. In this sense, the opposite could be:
* Closure: The final act, ending, or decision.
* Refusal: A rejection of the proposal or offer.
* Withdrawal: Taking back the overture or proposal.
It depends on the context! If you could tell me more about how "overture" is being used, I might be able to offer a more specific opposite.