"I think Craig has been fantastic for Newcastle and we wish him well at Celtic. He's only on loan until the end of the season so it remains to be seen what will happen with his situation," said O'Brien.
"People have different opinions but I personally rate him as a very good player and I'm sure he probably will be missed because there are very few players who are as quick as him in the Premier League."
O'Brien was himself the subject of transfer rumours in January but ended the speculation by signing a one-year extension to his contract at St James' Park.
"I was delighted to sign the extension.
"I know there's going to be competition for places but I'm not at Newcastle to make up numbers."
While he battles for a place at club level, O'Brien is also aware that his position at the heart of the Irish defence alongside Kenny Cunningham offers no grounds for complacency either.
And, to his mind, the two situations are inextricably linked. "My personal feeling is that it's important I play for Newcastle United to maintain the position I have in the Irish team," he said.
"It took me five years to get a start in a qualifying game and I'm determined to keep that position. There are Irish centre-halves doing very well at their clubs people like Richard Dunne at Manchester City and Gary Breen at Sunderland so I don't take it for granted that I'm going to be playing for Ireland.
"I respect the manager's decision whether it's for me or against me." Meanwhile, another Irish player aiming to impress both club and country tonight, if he gets the chance, is Manchester United's Liam Miller.
After arriving at Old Trafford from Celtic Park amid a blizzard of headlines and then making a promising start to his career with United, the 24-year-old from Cork now faces a daunting task to break into Alex Ferguson's first 11.
His troubles, he reckons, began with a bad performance in the cup.
"The whole team had a bad performance against Exeter, including myself, and it sort of put some of us on the back foot again," he said.
"We're back to the bottom of the squad rotation again. But you've just got to pick yourself up. And I believe I have recently.
"I'm doing well in training, I feel the benefit and I'm starting to get a look-in in the league again."
Miller said he never underestimated the size of the challenge at Old Trafford and has received nothing but encouragement from both Ferguson and Roy Keane. "I knew when I signed the players they have, I knew it was going to be tough.
"It's full of quality there isn't a bad player at United. So now it's just about me trying to improve every day.
"Hopefully I can. With Djemba Djemba leaving, it opened the gap a little for me to get back in but I know that I've got to work hard.
"I'm there to fight for a place and hopefully I'll do that.
"And if I can get in the team and show what I'm capable of doing at club level then it obviously gives me a better chance to play international football."
Miller also responded to Martin O'Neill's publicly expressed disappointment that the player had chosen to leave Glasgow for Manchester.
"I suppose he's entitled to his opinion," said Miller. "I have great respect for him as a manager and he's always been great for me, so I can understand his frustration at me leaving.
"But I feel I made the right decision and hopefully it will show in time."