2007 marked the first time he pitched more than four games in a year for Oakland; he an initial boost to their bullpen but missed the latter part of the season with an injury.

Obtaining false papers from a friend, Casilla began pitching under the name of Jairo Garcia, listing his birthday as March 7, 1983.

In 13 games (8 starts), he had a 2–1 record, a 2.44 ERA, 66 strikeouts, 17 walks, and 56 hits allowed in 59 innings pitched before getting promoted to the Vancouver Canadians of the Single-A short season Northwest League on August 24.

[4] On July 30, he was promoted to the Sacramento River Cats of the Triple-A Pacific Coast League (PCL); he made three appearances for them before getting called up to Oakland for the first time on August 9.

[3] Casilla made his Major League Baseball (MLB) debut that same day, holding the Minnesota Twins scoreless for the final 2 innings of an 8–2 victory.

[3] The Athletics called him up on June 17, and he faced the Philadelphia Phillies that evening in his lone appearance before getting returned to Sacramento four days later.

[2] Due to U.S. security concerns over the name change, Casilla was a month late reporting to spring training and was instantly assigned to work with the minor leaguers.

[2] He stepped into a depleted bullpen that had lost Huston Street, Justin Duchscherer, and Kiko Calero to the disabled list.

[2][6][7] He picked up his first MLB save on June 6, retiring Manny Ramirez on a fly ball to end the eighth inning and pitching a scoreless ninth in a 3–2 win over the Boston Red Sox.

[8] On June 19, he entered a game against the Cincinnati Reds with the bases loaded and struck out Brandon Phillips to keep a run from scoring.

[6] Umpire John Hirschbeck ejected Casilla from a game against the Tampa Bay Devil Rays on August 23 for throwing a pitch at Brendan Harris.

[1] Over the offseason, he made nine appearances for the Leones del Escogido of the Dominican Winter League, allowing no earned runs and collecting 4 saves.

[4] A member of Oakland's Opening Day roster again in 2009, Casilla posted a 1.59 ERA and .079 opponent batting average in his first 9 games.

[1] Visa problems delayed his return to the United States for a month, and he began the season with the PCL's Fresno Grizzlies.

[11] He made his Giants debut on May 21 against his former team in Oakland, striking out the only batter he faced (Daric Barton) in a 6–1 defeat.

[3][13] After he allowed three runs and blew a lead against the Arizona Diamondbacks on July 23, though the Giants still won the game 7–4, his ERA rose from 2.25 to 3.48.

[14] On August 10, he set a career high with three innings pitched, allowing no runs, though the Giants lost 8–6 to the Chicago Cubs.

[17] In Game 4 of the NL Division Series (NLDS) against the Atlanta Braves, with San Francisco leading 3–2, Casilla pitched a scoreless seventh inning.

[23] Casilla returned to the Giants in 2011, but after pitching on Opening Day (March 31), he was placed on the disabled list with inflammation in his right elbow.

[18] Against the Detroit Tigers in Game 2 of the 2012 World Series, Casilla relieved Madison Bumgarner to begin the eighth with the Giants leading 1–0.

[26] Prior to the 2013 MLB season, Casilla represented the Dominican Republic in the World Baseball Classic, pitching five scoreless innings as his country won the tournament.

[42] On June 30, Bochy removed the struggling Romo from the closer role on that date, announcing that the team intended to go to a closer-by-committee.

[18] He entered a tied Game 5 in the top of the ninth and loaded the bases by giving up a walk and a couple of hits, but Jeremy Affeldt got Oscar Taveras to ground out to end the inning with no runs scoring, and Travis Ishikawa homered in the bottom of the inning to give the Giants a 6–3 triumph, clinching a 4–1 series victory.

[46] He made only two appearances in the World Series against the Kansas City Royals, recording an out apiece in Games 2 and 3, both losses.

[48] On May 17, in a 9–8 win over the Cincinnati Reds at Great American Ball Park, Casilla entered the bottom on the ninth inning and struck out three batters on nine pitches, recording the save.

"[16] Steward wrote that "after that night, public opinion essentially forced Bochy’s hand not to use him again in an important situation, particularly at home.

[59] Visa problems caused him to be three weeks late to spring training, and he decided not to pitch for the Dominican Republic in the 2017 World Baseball Classic, even though he had initially been selected to the team.

[60] He was expected to compete for the closer's role, though Jane Lee of MLB.com wrote in January that the likely favorite was Ryan Madson, who had closed for the Athletics the season before.

[59] Oakland began the season without a closer named, though, and on April 11, manager Bob Melvin said in an interview that "It's gonna be [Sean] Doolittle or Casilla depending on the match-ups".

[1] Casilla worked with a prototypical power pitcher repertoire, chiefly throwing a low to mid-90s two-seam fastball and a hard-breaking slider.