The Yankees will be on the hook for the rest of Prado's contract, which will pay him $11 million in 2015 as well as in '16.
O'Brien, 24, was selected by the Yankees in the second round of the 2012 First-Year Player Draft and has put up monster power numbers in the Minor Leagues. Combined at Class A Tampa and Double-A Trenton this year he hit .267 with 23 doubles, 33 homers and 70 RBIs.
Ranked as the ninth-best prospect in the Yankees system by MLB.com, O'Brien was selected for this year's All-Star Futures Game.
O'Brien has hit 65 home runs in three seasons in the Yankees organization. Entering today, he was tied for second overall in the Minors in extra-base hits (58), third in home runs, fourth in total bases (229) and ninth in slugging percentage (.593).
Defense has been a question for O'Brien, who has seen time behind the plate, at first base, right field and third base during his time in the Yankees system.
Prado was acquired in the trade that sent Justin Upton to Atlanta prior to the 2013 season. He hit .278 last season and was hitting .270 heading into today.
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.