Cook, on his first career Opening Day roster, impressed manager Bob Melvin enough during the spring campaign -- opponents were just 2-for-23 against him -- to land a job as the primary right-handed setup option in the A's bullpen.
Things could change upon Joey Devine's return from the disabled list, but for now it's Cook and southpaw Brian Fuentes who will be looked upon for outs before closer Grant Balfour's arrival in the ninth.
"Those are the two guys I'm kind of targeting to have just before Grant," Melvin said. "You look at the personnel we have, and it really has to go on merit, based on what they've done in Spring Training."
The A's bullpen, which compiled a 3.74 ERA last year, is younger this season but well-balanced. In the two-game Opening Series in Tokyo, Melvin is equipped with three right-handers and three-left-handers, along with a length option in right-hander Graham Godfrey.