"I'm very proud of this," Severino said of the experience.
"It's a great opportunity and a huge blessing," added O'Brien.
The U.S. squad used O'Brien as a first baseman, the position du jour for the 23-year old ranked by MLB.com as the 20th-best prospect in the Yankees organization. Many believe O'Brien, now in Double-A, will ride his power to New York in the near future. It's the position that he'll play once he gets there that remains the unknown.
O'Brien, who has 29 homers, 17 doubles and 63 RBIs through 88 Minor League games this season, made the transition to first baseman only recently. He was drafted as a catcher in the second round of the 2012 First-Year Player Draft before beginning the move around the field last season.
He's spent time in right field and third base, the latter a position he enjoyed, but one where the learning curve was sharp. O'Brien committed 18 errors in 38 games at third last season. The bulk of O'Brien's playing time early this season came behind the plate, though his May promotion to Double-A Trenton left him behind top Yankees catching prospect Gary Sanchez.
About a month after that move to Trenton, the Yankees approached O'Brien about trying yet another new position. He had to borrow a teammate's glove for several days before getting his own ordered.
"I think for the time I've played it, I feel very, very comfortable there," O'Brien said about first base. "It feels very natural. I actually like everything that comes along with the position -- picking balls out of the dirt, having to hold guys on. I was kind of scared at first, because you think you're not going to get that much action. But I like it."
O'Brien still catches about once or twice a week, too, to keep that skill sharp. Though he may never become a plus defender at any position, the versatility O'Brien touts should make it easier for the Yankees to find a spot for him once they need his bat.
"At the end of the day, as long as I'm in the lineup, no matter where I am on the field, I'm going to be happy and play as hard as I can at that position," O'Brien said. "I think the biggest asset is I can catch. There are other guys who play first, right and third, but not many who also catch. I think me being able to catch also puts me up a notch."
O'Brien did not get to face fellow farmhand Severino on Sunday, as Severino was in and out before O'Brien took the field. Severino, a 20-year-old Dominican native, allowed one hit and struck out one in a 12-pitch fourth.
The right-hander flashed all three of the pitches that have the Yankees so excited about his future -- a fastball that sits 96-97 mph and can top out at 99, an improved changeup and a developing slider.
Since signing for $225,000 in 2012, Severino has made a quick climb through New York's system. He began the season with Class A Charleston before earning a June promotion to Class Advanced Tampa. Since the promotion, Severino has allowed three earned runs on 11 hits and six walks while striking out 28 in 20 2/3 innings, facing players mostly his senior.
"I have felt very strong," said Severino, listed as the Yankees' No. 9 prospect. "This year has been going well, and I'm looking forward to an even better second half of the season. My objective is to try and throw as many innings as possible, and to try improve my angle that I throw my changeup."