The news comes a day after Dino Maamria was dismissed by the League Two side after 10 months in charge.
Former Australia international Kewell has experience managing in the fourth tier of English football, having coached Crawley Town from 2017 to 2018 before spending a brief spell at Notts County.
Kewell, whose first coaching job was at Watford’s academy, lasted just 14 games in charge of Notts County before being sacked in November 2018.
Speaking after his appointment on Saturday, the 41-year-old told Oldham’s official club website: “I’m excited about the project and, don’t get me wrong, it’s going to take time.
“Because the one thing I think this club does need is stability and if we can get that, then we can start to grow things.
“We’ve got to get back to the basics and we’re having a ‘clean slate’ here now and I think the fans, myself, the owner, everybody wants to start seeing this club progress, because we all believe that’s there’s a big potential.”
Oldham finished 19th in League Two this season, with the term cut short due to the coronavirus pandemic.