The Thunder didn't make a trade deadline move, but that isn't stopping them from adding a player.
Butler, 33, was bought out by the Bucks on Thursday.
The Thunder have two open roster spots available and about $2.3 million in cash to spend under the tax line, though terms of the deal are unknown.
In 34 games with Milwaukee this season, Butler averaged 11.0 points and shot 39 percent from the field, with a true shooting percentage of 49 percent.
Butler will add to the Thunder's already impressive depth, providing a veteran wing scoring presence off the bench to spell Kevin Durant.