Republic of Ireland international O'Brien has made close-on 500 career appearances with Bradford, Newcastle, Portsmouth and Wanderers.
But he revealed: "Although I am delighted to be back, I would have been even happier if I had been given the chance of playing first-team football for Leeds all those years ago.
"But I got released at 14 without getting the chance to move up the ladder with Leeds.
"Paul Hart was the youth team coach and he put together an excellent team with the likes of Harry Kewell and Jonathan Woodgate.
"Competition was very high, and it was deemed too high for me. But it is ironic how, 17 years later, the wheel has turned full circle. I'm determined to make up for lost time."