Righthander Brandon McCarthy reached agreement with the Diamondbacks on a $15.5 million, 2-year contract, according to a source.
The deal is pending a physical, which will be especially important for McCarthy, who was Oakland's opening day starter last season.
McCarthy was hit in the right side of the head by a line drive off the bat of the Los Angeles Angels' Erick Aybar on Sept. 5. The 29-year-old pitcher sustained an epidural hemorrhage, brain contusion and skull fracture, then underwent a 2-hour surgery.
Last month, McCarthy spent 2 days undergoing extensive evaluations by renowned concussion expert, Dr. Michael Collins, at the University of Pittsburgh. He was cleared by Collins to begin working out and resuming his regular offseason routine.
Oakland's medical staff initially warned that McCarthy's situation was very much "life-threatening."
McCarthy said in November that his concussion symptoms had subsided, but he still has to use caution when exerting himself. McCarthy went 8-6 with a 3.24 ERA this year in his sixth big league season and second with the A's. He was 9-9 with a 3.32 ERA in 2011.
In other transactions
* Righthander Dan Haren agreed to a 1-year contract with the Washington Nationals. Haren, 32, was a free agent after going 12-13 with a 4.33 ERA in 30 starts for the Los Angeles Angels in 2012.
* Speedy outfielder Brett Gardner and the New York Yankees agreed to a $2.85 million, 1-year contract that avoids salary arbitration. Gardner was limited to 31 at-bats last season because of a right elbow injury.