Hartlepool were on the brink of saving themselves after coming from behind against title-chasing Doncaster thanks to two goals from teenager Devante Rodney
But their 2-1 win at the Victoria Park counted for nothing in the end as O'Brien snatched a 2-1 victory over Notts County to ensure Newport, who were 11 points adrift of safety on March 4, completed their great escape
A see-saw evening saw Doncaster go ahead through Andy Williams, and moments later Newport took the lead in their match through a Mickey Demetriou penalty
Yet Jorge Grant levelled for County to give Hartlepool hope before Rodney, an 18-year-old yet to start a senior match, let alone score a goal, appeared to have written his name in Hartlepool history with a second-half double
But with 60 seconds remaining O'Brien chested the ball down and volleyed a winner which saved the Welsh side and ended Hartlepool's 96-year stay in the Football League
Doncaster's defeat meant they missed out on the title, which eventually went to Portsmouth after a thumping 6-1 win over Cheltenham.
An own-goal from Dan O'Shaughnessy, two from Kal Naismith and further strikes from Kyle Bennett, Jamal Lowe and Gareth Evans made it a day of celebration at Fratton Park.
Plymouth had started the day on top, but they fell behind to a Shaun Pearson goal after 37 seconds at Grimsby and could only muster Jimmy Spencer's equaliser in a 1-1 draw.
There were seven teams in with a shout of claiming the last two play-off spots, but in the end it was as you were as Carlisle and Blackpool finished sixth and seventh.
Carlisle made hard work of things, however, as they twice trailed Exeter and needed two Danny Grainger penalties and Jamie Proctor's winner to snatch a 3-2 victory.
The two teams will now meet in the play-offs, while Blackpool, 3-1 winners against already relegated Leyton Orient, will face Luton.
Colchester finished eighth after beating Yeovil 2-0, Wycombe beat fellow hopefuls Cambridge 1-0, Mansfield drew 2-2 at Crawley and Stevenage's hopes were ended by a 3-0 defeat by Accrington.
Away from the drama Luton limbered up for the play-offs with a 3-1 win over Morecambe and Crewe thumped Barnet 4-1.
Source : PA