O'Brien, ranked by MLB.com as the D-backs No. 7 prospect, made a lot of progress as a catcher early in Spring Training, but was derailed toward the end of camp by some throwing issues.
"He's playing the outfield, he'll play a little first," D-backs manager Chip Hale said. "I think they are waiting about a month to get him back in there. Just to kind of clear his head and let him get off to a good offensive start."
O'Brien's days behind the plate, though, are not necessarily over.
"He had come a long way catching-wise and gained the trust of the pitchers," Hale said. "I really feel like at some point there could be a chance he'll get back there again, and it will be a quick [transition]."
Yasmany Tomas, who was signed out of Cuba by the D-backs this past winter to a six-year $68.5 million contract, is hitting .190 for Reno with a home run in his first 21 at-bats, while playing right field.
"I look at video of Yasmany almost every day," Hale said. "Kind of like Spring Training, he's had some really good at-bats and some where the pitchers got the best of him, but he's playing really hard. He's playing very well in the outfield. I've heard his defense has been great, and he hit a home run the other day, I saw that. It's tough. He's down there, and he's battling with the rest of the guys. He's been a really good teammate."