"He's going to play," D-backs manager Chip Hale said. "He's going to play left field, he's going to play close to every day, five out of six, five out of seven. He's going to get an opportunity, he's earned it."
O'Brien batted .330 with 17 home runs and 52 RBIs in 51 games for Reno. His 17 homers leads the Minor Leagues, as does his 142 total bases.
O'Brien was previously promoted on May 27, but he only had one at-bat -- striking out on three pitches pinch-hitting against the Padres -- before being sent back down to Triple-A three days later.
O'Brien, ranked Arizona's No. 7 prospect by MLBPipeline.com, was a September callup in 2015. He made only one start and went 4-for-10 with a homer in eight games.
O'Brien was in the lineup for Friday's 8-6 loss the Marlins, batting fifth and playing left field. He finished 0-for-4.
"It's definitely pretty exciting, it's something that I've worked for and I'm just happy to be here and do whatever I can to help the team win," O'Brien said. "This is where the best of the best are for a reason. I'm not saying that I'm the best, but I think I definitely, over the last couple years, put together some good seasons and I'm looking forward to hopefully being an everyday big-leaguer."
The D-backs have seven outfielders on their active roster, not including backup catcher Chris Herrmann, who can also play out there. Hale said O'Brien's promotion likely will cut into the playing time of fellow corner outfielders Yasmany Tomas and Brandon Drury.
Center fielder Chris Owings (foot soreness) was out of the lineup for the fourth straight game on Friday against the Marlins. If Owings avoids a trip to the disabled list, it could be one of Arizona's other corner outfielders being sent down to make room for Saturday starter Zack Godley.
To make room for O'Brien on the active roster, the D-backs optioned Edwin Escobar to Reno. Escobar made two starts, going 0-2 with a 14.14 ERA. He was moved to the bullpen on Monday, but never made a relief appearance.
Hale said Escobar will join the Triple-A starting rotation, but could be back to the Majors in the bullpen later this season.