It took some foresight to set things up such that they could absorb Durant. They didn't extend Barnes last year because they wanted to keep the cap space open. They signed guys that would expire this year, like Speights and Barbosa and Livingston (though they'll likely keep Livingston if they can find a buyer for Bogut). Even Bogut is an expiring contract next year, so he shouldn't be super hard to move. Obviously, it took some luck too--they got Curry on a bargain deal due to signing him when his health-related value was lowest, but that also proved to be a smart gamble.
I don't think "deciding to sign Durant if he's willing" counts as brilliant, but they set this up intelligently and obviously made a great sales pitch to a guy who said he was "90% sure" he was returning to Oklahoma City.