Worthington moved to Oldham in 2009 under then boss Dave Penny, but was transfer-listed by new manager Paul Dickov last summer.
He will go straight into City's squad for Tuesday night's game at Crewe.
Worthington made 17 appearances for Oldham last season, but has only managed three games this campaign, all on loan at non-league Fleetwood.
His arrival will help ease the injury crisis Bradford are currently enduring in midfield.
Former Wycombe man Tommy Docherty will have an ankle operation on Thursday, while Lee Hendrie left the club at the start of the year.
Meanwhile Worthington's former Huddersfield team-mate Michael Flynn has only just returned from his various injury problems.
Flynn has managed just 27 minutes of football all season but is expected to be on the bench against Crewe, and before Worthington's arrival City had just David Syers and Norwich loanee Tom Adeyemi as fully fit midfielders.
Source: BBC Sport