"I think we were just pretty happy with what we're seeing from the four that are left, and we just felt like it's time for [O'Brien] to go get more at-bats and get ready for the season," D-backs manager Chip Hale said.
O'Brien played first base, third base, catcher and right field during his time in the Yankees' system, and Hale said the team had not determined where he would play this year in the Minors.
"We're going to discuss that," Hale said. "We want to make sure we put him in the spot, if he's a potential 30-40 home runs guy with 100 RBIs, we need him to play a lot. So is that at catcher? Or would it be somewhere else? So we'll make a decision as an organization and with him, get his input."
The other six non-roster players reassigned to Minor League camp Sunday were Jake Barrett, Danny Dorn, Nick Evans, J.C. Ramirez, Jamie Romak and Danny Worth.
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.