Brooks allowed one run in 5 1/3 innings as Oakland completed its first sweep of the three-exhibition-game Bay Bridge Series since 1989. Showers prompted umpires to call the game after six innings.
Brooks, who struck out five and walked two, said that he felt "excited and blessed" to make his first Opening Day roster after nine professional seasons.
Said A's manager Bob Melvin, "There was some pressure on him. He was trying to get a job and he pitched really well. So we gave him a job."
The fifth starter's role often is considered an afterthought. That's not the case with the A's, who will play 18 consecutive regular-season games before receiving a day off. Melvin wants his starters to consume as many innings as possible to keep his bullpen fresh during this stretch.
The Giants' best chance to score directly off Brooks occurred in the fifth inning, when Steven Duggar's drive to left field with one out -- and Gerardo Parra on first base -- died at the wall. A run was charged to Brooks when Connor Joe, whose single chased the right-hander, came around to score in the sixth.
"I was just trying to pound the zone and throw strikes," said Brooks, who the A's purchased from Colorado last September. "I know they're going to come out swinging, so I tried to force early contact."
Brooks' 4.40 ERA in three spring starts was deceiving. He threw 11 consecutive scoreless innings spanning three starts, including Tuesday, since yielding six runs over two innings in his Feb. 24 Cactus League debut against Kansas City.
"I got more confidence in throwing strikes and challenging hitters," said Brooks, who attributed most of his effectiveness against the Giants to his sinker-slider combination.
Brooks' most significant competition was right-hander Chris Bassitt, who went on the injured list with a bruised shin, and left-hander Jesus Luzardo, who will be sidelined for at least four weeks with an ailing shoulder.
The A's finished their exhibition campaign with a 5-0 record against the cross-bay rival Giants, marking the first time since 1998 that they went unbeaten and untied in preseason against San Francisco.