The Angels traded starter Joe Saunders, two minor leaguers and a player to be named to the Diamondbacks for Haren on Sunday.
The righty made his last start July 21, putting him on schedule for a Monday start. The Angels bumped Joel Pineiro to give Haren his first American League start since 2007 with the A's.
This isn't Haren's first blockbuster trade. He was dealt by the Cardinals to the A's in 2004 in a trade that brought Mark Mulder to St. Louis. In 2007, the A's sent him to Arizona in an eight-player swap.
Haren went 43-34 with a 3.64 ERA with one All-Star appearance in three seasons in Oakland. He was 37-26 and made the All-Star Game two more times in his two-plus seasons with the D-backs.
Meanwhile, thanks to two upcoming off-days, the Angels will go with a four-man pitching rotation between now and Aug. 6, manager Mike Scioscia said Monday.
Haren will start Monday against the Red Sox. He'll be followed in the rotation by Jered Weaver, Pineiro and Ervin Santana. The Angels are hopeful that Scott Kazmir will return from the disabled list in time to pitch Aug. 6 at Detroit.
"I think it's important for us to get that solid fifth guy back and, hopefully, Scott will be ready," Scioscia said.
Open dates Thursday and Aug. 2 allowed the Angels to go with the four-man look as they fight to stay in contention. They entered Monday seven games behind the first-place Texas Rangers.
The Angels also optioned their interim fifth starter, Trevor Bell, to Triple-A Salt Lake and promoted reliever Michael Kohn. The team also recalled pitcher Matt Palmer from rehab and optioned him to Triple-A.
Kohn is set to make his major league debut after posting a 1.95 ERA this season with Salt Lake. He was a 13th-round pick in the 2008 draft.
Bell was sent down to continue working regularly after losing his start Sunday at Texas. The Angels only need four starting pitchers for the next two weeks because of off days.
Palmer has been on the disabled list since May 11 with a sprained right shoulder. He began a rehab assignment July 12.
Information from ESPNLosAngeles.com's Mark Saxon and The Associated Press contributed to this report.