O'Shea said that Keane would be a big loss to the national team's coaching staff, but agreed that the Celtic job would be hard for him to turn down.
"It looks very much like it now," Sky Sports News quotes O'Shea as saying. "Slowly but surely, it has snowballed into having talks with Celtic, so I would be very surprised if it doesn't get across the line. If it does happen, it's one of the clubs around the world that was going to entice him back into management and away from his current job.
"It's something I'm sure he would really, really enjoy and look forward to and relish the challenge of managing a fantastic club like Celtic. But let's wait and see if it does happen.
"As [Martin O'Neill] said, we would be very disappointed because we know what a fantastic character he is to have fighting for us on our side going into matches."
O'Shea played with Keane at Manchester United.