Oakland secured its first postseason berth since 2006, but the Athletics remain in pursuit of a bigger prize.
Coco Crisp had two hits, including a go-ahead RBI double in the fifth inning, and two runs scored in the A's 4-3 win over the Texas Rangers in the opener of a regular season-ending three-game series.
The hard-charging A's, who have won seven of their last nine games, need to sweep Texas in order to win the American League West crown.
"Absolutely amazing," Oakland manager Bob Melvin said. "We don't get this done unless everybody believes in everybody and everybody plays unselfishly."
Crisp returned to the lineup Friday after missing nine starts with an infection in both eyes. He's hitting .529 (9-for-17) with seven runs and two RBIs in four games since coming back.
Jarrod Parker (13-8) fanned six and surrendered three runs on six hits and two walks over six-plus frames to pick up the win.
Pitching to chants of "Let's go Oakland!," Grant Balfour struck out the side in the ninth and notched his 23rd save of the season.
The A's players then brought bottles of champagne onto the field and began spraying it into the crowd.
Oakland's victory knocked the Los Angeles Angels of Anaheim and Tampa Bay Rays from playoff contention.
"They play with no conscious. They're not afraid of nobody right now," Rangers manager Ron Washington said.
Martin Perez (1-4) allowed four runs on six hits and two walks in four-plus innings for Texas, which holds a one-game lead over Oakland and is tied with the New York Yankees for the best record in the American League. The Yankees defeated the Boston Red Sox earlier on Monday.
With the game knotted at 2-2, the A's went ahead following a two-run fifth.
Consecutive doubles by Adam Rosales and Crisp to begin the frame brought home a run.
Scott Feldman then relieved Perez and Crisp stole third before Yoenis Cespedes walked to put runners on the corners. Pinch-hitter Brandon Moss lifted a sacrifice fly to center field, which plated Crisp and gave the A's a 4-2 edge.
Texas cut the deficit in half in the seventh when Mike Napoli cracked a towering leadoff homer just over the wall in left-center field.
Ryan Cook fired a 1-2-3 eighth.
Earlier, the A's jumped out to an early lead after plating two in the opening frame.
Crisp and Jonny Gomes tallied back-to-back singles, which preceded Cespedes' double-play groundout. Chris Carter walked to put runners on the corners and Josh Reddick reached out and slapped a single to center field to bring home Crisp.
After Josh Donaldson walked to load the bases, Carter scored on a Perez balk.
Texas got a run back in the third and tied the contest in the fourth.
Elvis Andrus ripped an RBI single in the third and Michael Young smacked a one-out, opposite-field solo shot to right-center field in the fourth, which made it 2-2.
The 2006 American League West-winning A's went 93-69 and were swept by the Detroit Tigers in the American League Championship Series ... Oakland was 2- for-8 with runners in scoring position, while Texas finished 2-for-7 with RISP ... Napoli's homer was his 24th of the year ... Parker matched fellow starter Tommy Millone's Oakland rookie record for wins.