O’Brien moved to Bradford earlier this season and made 15 appearances before Magpies made him his second signing of the January transfer window to the league two side,Notts County.
The 31-year-old has made more than 380 senior appearances throughout his career after his breakthrough at Scottish giants Celtic.
Manager Neal Ardley said: ‘To get Jim in on a permanent deal is massive because it frees up loan spaces for when we do work elsewhere.
‘We’ve been tracking him for a while and we’ve worked so hard to bring him in. I think that feeling of being really, really wanted – a big transfer target for someone – has put things in our favour.
‘There are so many positives to this signing and to get him to leave a League One side to come to Notts County is huge.’