O'Brien becomes the latest player to leave the Premiership strugglers after Dean Windass and David Hopkin quit the club last week.

The 21-year-old, currently away on international duty, had a long-standing gentleman's agreement with Bradford that they would not stand in his way if the right offer came along.

And O'Brien, who came up through the Bantams ranks as a trainee, expressed his gratitude to the club for respecting the accord.

He told City's official website: "I shall be forever grateful to Bradford for giving me this opportunity.

"And I would like to thank the chairman for keeping his word and thus giving me the chance to fulfil my international ambitions.

"I would also like to thank the supporters of the club who have always been fantastic towards me. Bradford City will always be close to my heart."

Ironically, O'Brien's first chance to pull on a black-and-white shirt could come against his former club when Newcastle travel to Bradford on 31 March.

The signing comes after the news that Bobby Robson has been given the go-ahead to bring Leeds striker Michael Bridges back to Tyneside.

Robson is reported to be lining up a �7m bid for Bridges following vice-chairman Douglas Hall's pledge to loosen the purse strings at St James' Park.

The former Sunderland frontman has been sidelined for most of the season with a serious ankle injury and faces a difficult task to get back into the Leeds first team.

But Robson is determined to have three quality strikers in his squad and will wait for Bridges to prove his fitness.

Newcastle have been promised �100m over the next two years by Hall which will see the club become a major player in the transfer market again.

And the son of former chairman Sir John Hall is not just setting his sights on luring the best English talent up north.

"For the first time in a long time, there's cash in the bank," said Hall.

"Rivaldo was always my dream and Newcastle should attract players of his quality. We can attract any player from anywhere in the world.

"We plan to invest heavily in the future to give them a team to be proud of again. This is where it all takes off again."