The club announced on Friday that Maamria would be leaving the club after 10 months in charge where he won just nine matches out of 32.
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Kewell has been given a one-year deal by the club with an option to extend it.
"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," Kewell said in a statement.
"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."
The 41-year-old made nearly 400 appearances over an 18-year playing career which included time at Liverpool, Leeds United and Galatasaray.
In 2001, he received a Ballon d'Or nomination and won the Champions League with Liverpool in 2005.
Oldham finished 19th after the League Two season was curtailed by the coronavirus.
Kewell is the club's seventh manager, including former Manchester United midfielder Paul Scholes who lasted just 31 days, since Abdallah Lemsagam became owner in 2018.