The experienced centre-half is poised to link-up with United today in a month-long deal to help shore up a defence that has conceded 15 goals in its last five outings.
Although the current agreement sees him link-up with the Whites until the end of November, it is thought if his spell on the other side of the Pennines proves successful, it could lead to a permanent move when the transfer window re-opens in January.
Bolton boss Owen Coyle had previously rejected Middlesbrough's offer to take the former Republic of Ireland international on loan, but has since changed his mind after Sam Ricketts showed himself to be a capable understudy to first-choice centre-half pairing Zat Knight and Gary Cahill.
O'Brien has played just 42 minutes of Premier League football this season and missed his big chance to impress after limping off after just 18 minutes of the 1-1 draw against Aston Villa with a hamstring injury.
The Harrogate-born 31-year-old is in the last year of his contract and hinted over the summer that he would be prepared to drop a division to secure a permanent move away from the Reebok.
Although the current agreement sees him link-up with the Whites until the end of November, it is thought if his spell on the other side of the Pennines proves successful, it could lead to a permanent move when the transfer window re-opens in January.
Bolton boss Owen Coyle had previously rejected Middlesbrough's offer to take the former Republic of Ireland international on loan, but has since changed his mind after Sam Ricketts showed himself to be a capable understudy to first-choice centre-half pairing Zat Knight and Gary Cahill.
O'Brien has played just 42 minutes of Premier League football this season and missed his big chance to impress after limping off after just 18 minutes of the 1-1 draw against Aston Villa with a hamstring injury.
The Harrogate-born 31-year-old is in the last year of his contract and hinted over the summer that he would be prepared to drop a division to secure a permanent move away from the Reebok.