Coming off a 75-87 record last season, expectations for the A's were modest. Their biggest offseason move was acquiring outfielder Stephen Piscotty in a trade with the St. Louis Cardinals.
The combination of a loaded bullpen, excellent defense and development from position players such as Matt Olson and Matt Chapman led to Oakland's resurgence in the American League West.
After falling to 36-36 with a 6-5 loss to the Los Angeles Angels on June 17, the A's hit their stride and didn't look back. They have gone 58-26 since that defeat and are still within striking distance of the Houston Astros to win the division.
Per FanGraphs' Craig Edwards, All-Star closer Blake Treinen is having a historic season out of the Oakland bullpen:
"The combination of Treinen's good pitching and Oakland's defense—and, perhaps, a little bit of luck—means that only four relievers have ever posted an ERA- lower than Treinen's current figure of 21: Zach Britton, Dennis Eckersley, Jonathan Papelbon, and Fernando Rodney. When you combine that run-prevention with a ton of high-leverage situations, you get one of the top Win Probability Added (WPA) seasons of all time."
As Sullivan noted, the Athletics defense has also been crucial to their success. They rank third in MLB in FanGraphs' defensive value (36.4) and 10th in defensive runs saved (27). For comparison, they finished 27th in defensive runs saved and last in defensive value during the 2017 campaign.
Adding to the stunning nature of Oakland's accomplishments this season, Sean Manaea is the only pitcher in the starting rotation with at least 150 innings pitched. Unfortunately, the southpaw had shoulder surgery on Sept. 19 and will miss all of next season.
Through all of the rotation turnover, the A's haven't skipped a beat. They are back in the playoffs for the first time since 2014 and will be a dangerous opponent because of their lineup and bullpen depth.