On December 14, 2007, he was traded by Diamondbacks with Dana Eveland, Greg Smith, Aaron Cunningham, Carlos González and Chris Carter to the Oakland Athletics for Dan Haren and Connor Robertson.

In 2008, Anderson was a combined 11–5 with a 3.69 ERA and 118 strikeouts in 20 games (19 starts) between the Stockton Ports in the California League and the Double-A Midland RockHounds of the Texas League.
In his debut for Midland, he struck out 12 batters and gave up two runs in six innings for the win.
He was selected to pitch in the All-Star Futures Game played at Yankee Stadium during the All-Star break and was named to the U.S. Olympic Team in Beijing.
He pitched seven innings to collect the win in the bronze medal game against Japan.

Anderson was a highly touted young prospect; in 2008 he was ranked as the 36th-best prospect in the Major Leagues, and in 2009 he was ranked the top prospect in Oakland's system and the #7 prospect overall by Baseball America.

Anderson was promoted to the Athletics as part of their opening day roster for 2009 and made his Major League debut on April 10, 2009 against the Seattle Mariners, allowing five runs in six innings to take the loss.
He picked up his first win when he allowed only two earned runs in six innings against the Tampa Bay Rays on May 20.
On July 6, he pitched a shutout against the Boston Red Sox striking out 9 batters, a career high.
It was his first career shutout, and his first career complete game.

On September 24, 2009, Anderson broke the Oakland Athletics single-season rookie strikeout record (previously set by Rick Langford in 1977), while racking up 6 strikeouts in 5​²⁄₃ innings versus the Texas Rangers.
He finished the 2009 season with an 11–11 record, posting a 4.06 ERA.
He led the team in wins (11) and shutouts (1) and tied with Brett Tomko in strikeouts (150).

The Athletics signed Anderson to a four-year, $12.5 million, contract extension the start of the 2010 season, which he began by throwing six shutout innings against the Mariners.
He allowed 3 hits, struck out 4 and walked 1, getting the win.
In 19 starts in 2010 he was 7-6 with a 2.80 ERA.
In 2011, after a June 5 outing, he was placed on the disabled list, eventually undergoing Tommy John surgery which required 13 months of rehabilitation.
In 13 starts before the injury he was 3-6 with a 4.00 ERA.

Anderson returned August 21, 2012 for the Athletics, allowing one run in seven innings for a win against the Minnesota Twins.
However, he suffered an oblique strain during a start against the Detroit Tigers toward the 4th inning and was later pulled out, taking the loss.
He spent the remainder of the 2012 season on the disabled list with the injury, and had a 4-2 record in six starts that season.
Despite his limited action during the regular season, the Athletics put him on the playoff roster for the 2012 American League Division Series against the Detroit Tigers.
Anderson pitched six scoreless innings in a 2–0 victory in game three to keep the Athletics alive, though they eventually lost the series in five games.

On February 28, 2013, Anderson was named Opening Day starter for the A's for the 2013 season.
He went up against Mariners ace Félix Hernández and came away with the loss.
 He left his start on April 19 with what was called a sprained ankle but was later determined to be a stress fracture in his right foot.
He originally hoped to return to the rotation in July, but the injury was more serious than previously thought and he was placed on the 60-day disabled list.
He was eventually activated from the disabled list on August 28, though the Athletics chose to use him out of the bullpen the rest of the season.
He appeared in 16 games that season, only five of which were starts and was 1-4 with a 6.04 ERA.
He also appeared in relief in game four of the 2013 American League Division Series against the Tigers, allowing the two winning runs to score on a walk, wild pitch and double.
On December 14, 2007, he was traded by Diamondbacks with Dana Eveland, Greg Smith, Aaron Cunningham, Carlos González and Chris Carter to the Oakland Athletics for Dan Haren and Connor Robertson.
In 2008, Anderson was a combined 11–5 with a 3.69 ERA and 118 strikeouts in 20 games (19 starts) between the Stockton Ports in the California League and the Double-A Midland RockHounds of the Texas League. In his debut for Midland, he struck out 12 batters and gave up two runs in six innings for the win. He was selected to pitch in the All-Star Futures Game played at Yankee Stadium during the All-Star break and was named to the U.S. Olympic Team in Beijing. He pitched seven innings to collect the win in the bronze medal game against Japan.
Anderson was a highly touted young prospect; in 2008 he was ranked as the 36th-best prospect in the Major Leagues, and in 2009 he was ranked the top prospect in Oakland's system and the #7 prospect overall by Baseball America.
Anderson was promoted to the Athletics as part of their opening day roster for 2009 and made his Major League debut on April 10, 2009 against the Seattle Mariners, allowing five runs in six innings to take the loss. He picked up his first win when he allowed only two earned runs in six innings against the Tampa Bay Rays on May 20. On July 6, he pitched a shutout against the Boston Red Sox striking out 9 batters, a career high. It was his first career shutout, and his first career complete game.
On September 24, 2009, Anderson broke the Oakland Athletics single-season rookie strikeout record (previously set by Rick Langford in 1977), while racking up 6 strikeouts in 5​²⁄₃ innings versus the Texas Rangers. He finished the 2009 season with an 11–11 record, posting a 4.06 ERA. He led the team in wins (11) and shutouts (1) and tied with Brett Tomko in strikeouts (150).
The Athletics signed Anderson to a four-year, $12.5 million, contract extension the start of the 2010 season, which he began by throwing six shutout innings against the Mariners. He allowed 3 hits, struck out 4 and walked 1, getting the win. In 19 starts in 2010 he was 7-6 with a 2.80 ERA. In 2011, after a June 5 outing, he was placed on the disabled list, eventually undergoing Tommy John surgery which required 13 months of rehabilitation. In 13 starts before the injury he was 3-6 with a 4.00 ERA.
Anderson returned August 21, 2012 for the Athletics, allowing one run in seven innings for a win against the Minnesota Twins. However, he suffered an oblique strain during a start against the Detroit Tigers toward the 4th inning and was later pulled out, taking the loss. He spent the remainder of the 2012 season on the disabled list with the injury, and had a 4-2 record in six starts that season. Despite his limited action during the regular season, the Athletics put him on the playoff roster for the 2012 American League Division Series against the Detroit Tigers. Anderson pitched six scoreless innings in a 2–0 victory in game three to keep the Athletics alive, though they eventually lost the series in five games.
On February 28, 2013, Anderson was named Opening Day starter for the A's for the 2013 season. He went up against Mariners ace Félix Hernández and came away with the loss.  He left his start on April 19 with what was called a sprained ankle but was later determined to be a stress fracture in his right foot. He originally hoped to return to the rotation in July, but the injury was more serious than previously thought and he was placed on the 60-day disabled list. He was eventually activated from the disabled list on August 28, though the Athletics chose to use him out of the bullpen the rest of the season. He appeared in 16 games that season, only five of which were starts and was 1-4 with a 6.04 ERA. He also appeared in relief in game four of the 2013 American League Division Series against the Tigers, allowing the two winning runs to score on a walk, wild pitch and double.
Brett Anderson (baseball)