Latics chief executive Alan Hardy said: "He was officially released by Sheffield United this morning [Wednesday] and we've moved quickly.
"We've been able to conclude the deal following the completion of a successful medical."
Oldham want to make one more addition - a goalkeeper - to add to the signings of Rob Scott, Paul Warne, Richard Butcher, Terrell Forbes, Paul Edwards, Richie Wellens, Gareth Owen and Chris Porter.
Meanwhile defender Dean Holden has joined Peterborough.