O'Hara was sent from training camp at the University at Albany to the Hospital for Special Surgery in New York on Tuesday for an MRI.
The examination revealed no new injury to the ankle. The swelling, however, is the result of a chronic condition in the joint.
Doctors and trainers will try to reduce the swelling so O'Hara can regain mobility, the Giants said.
O'Hara participated in conditioning drills Sunday morning and a light workout that afternoon. He didn't appear to hurt the ankle at any time.
Swelling developed in O'Hara's ankle late Sunday, and he has missed four practices over the past two days at the University at Albany.
O'Hara has started all but five regular-season games at center since joining the Giants as a free agent in 2004. Four of the games he missed came in that first season, with the other happening in 2006.
Adam Koets, who has played in three regular-season games in his first three years in the league, has taken over at center in O'Hara's absence.
"Everyone wants to play," Koets said between practices Tuesday. "It is an opportunity for me, and I just have to make the most of it."
Drafted as a tackle out of Oregon State in sixth round in 2007, Koets has never started an NFL game. He appears to have put on muscle during the offseason, though, and seemingly hasn't made the blatant errors that had coaches all over him earlier in his career.
"I kind of realized I have been here three years and it's time for me to step up and play," said Koets, who can play any position on the line. "It's an opportunity for me I have to show what I can do."
Veteran Rich Seubert, who is fighting for his job at left guard, also has seen more work at center the past two days. He downplayed O'Hara's ankle injury Monday, saying his teammate will be fine.
"We all like to take a couple of days off from practice," Seubert said. "I did last year for a couple of weeks. It's O-Line humor. He'll be fine. One thing you don't have to worry about, unless we're at the hospital laying down and they are tying you down, you're going to try to get out there as soon as you can. We all know Shaun would rather be out there practicing."
Manningham made 57 catches for 822 yards and five touchdowns last season, while Nicks had 47 receptions for 790 yards and six TDs. Steve Smith led the team with 107 catches for 1,220 yards and seven TDs.
Copyright 2010 by The Associated Press