Royals manager Ned Yost calls O'Brien's power "prodigious" and recalled something D-backs chief baseball analyst Tony La Russa once told Royals hitting coach Dale Sveum about O'Brien.
"He told Dale that he'd seen two guys with that kind of power," Yost said, "and that was Mark McGwire and Peter O'Brien."
The second thing that will grab your attention about O'Brien is that he is fluent in Spanish.
"It does surprise people a little bit," O'Brien said. "Someone will say something in Spanish and I'll answer back, and it kind of freaks everyone out. With a last name like O'Brien, it really surprises people."
O'Brien learned Spanish from his mother, Mercedes, a former professional ballerina who defected from Cuba. Mercedes met Terry O'Brien, an Irishman from Michigan, at a show in Miami and the two began dating, then eventually married in 1989.
Peter O'Brien grew up learning English and Spanish.
"It helps being part of the clubhouse, to be able to speak Spanish," O'Brien said.
Of course, O'Brien's primary skill involves hitting a baseball. He already has caught the attention of his Royals teammates, who are wary of being in the same batting practice group for fear of looking like weaklings.
O'Brien admits he loves that kind of attention when he's crushing balls to all fields.
"It's fun to get some comments from the guys," O'Brien said. "It's really cool. I love hitting, so it's nice to get that feedback."
O'Brien homered in his first Spring Training game with the Royals on Saturday. But he will have an uphill climb to crack the 25-man roster as only precious few spots are up for grabs, and those will be highly contested.
But O'Brien, who was acquired in a trade with the D-backs in January for Minor League right-hander Sam Lewis, remains confident he belongs in the big leagues.
O'Brien, 26, played in 36 games for the D-backs over the past two seasons, hitting .176 with six home runs.
"I think the main thing is to make the game as simple and as easy as possible," O'Brien said. "I'd say it's more mental than anything for me. My mechanics are there and I'd say I have a great swing. And everything else I do really well.
"But what I need to do is try and slow the game down and let my tools take over. I have the tools to be here."