Play was stopped immediately and O'Nora was treated on the steps of the Royals' dugout by team trainers. He left under his own power, aided by the trainers.
Olivo's batted ball was fielded by Colorado Rockies shortstop Troy Tulowitzki, who threw to first base for a routine out. Meantime, Olivo was left with the bat handle and O'Nora was staggered.
"I just feel sorry for him, I feel bad," Olivo said. "I looked back and saw the blood come out and I just felt so bad. I saw [Jose] Guillen going in with the towels and thought, 'Oh, no.'"
Olivo was using a maple bat when the injury occurred. He said he switched to ash bats for the rest of the game.
Olivo, the Royals' designated hitter, went to a room in the stadium while O'Nora was being treated to tell him how sorry he was about the accident.
"It's not my fault, but I see that and my heart is too sensitive," Olivo said.
The game was stopped for 13 minutes while O'Nora was treated, and first-base umpire Paul Nauert put on home-plate protective gear to take over for O'Nora.
Ironically, the injury took place on a day that Major League Baseball's Safety and Health Advisory Committee held a conference call to discuss what to do about injuries caused by broken bats.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.