The 27-year-old expected to be stuck on the treatment table for the rest of the season and was forced to sit out fellow Lion Brian O'Driscoll's Ireland farewell earlier this month, with Chris Henry filling the No.7 shirt.
However, he is now hoping to be back on the training pitch within a fortnight - though O'Brien is not expecting to walk straight back into Joe Schmidt's side.
"It's going very well. I'm back ahead of schedule and I'm just looking forward to starting my pre-season in two weeks' time," he told KCLR FM.
"That'll be tough work for six or eight weeks, but it's something I'm looking forward to.
"I'm just looking forward to getting fit again and getting cracking into the year when I do get going.
"We are after taking a step forward and we're playing a good brand of rugby. There's exciting times ahead I think.
"There is massive competition there in the back-row now and Chris had a great tournament, so fair play to him.
"I'll have to fight for my place when I get back and I look forward to that."