Joining originally on transfer deadline day in the summer, the highly-rated young defender has impressed during his first spell in the EFL.
On keeping Clarke in his defensive ranks, Latics manager Harry Kewell said; "Harry has been an instant success since coming in at the start of the season.
"For a young man he's a very taletned individual and I'm sure the supporters have enjoyed seeing the impact he has made in the heart of defence already.
"It was an incredibly easy decision to extend his loan and I'm thankful Arsenal saw that this move has already shown it's benefits to one of their exceptionally gifted players."
And Clarke himself was pleased with the news, telling Oldham's website: "I'm buzzing to be able to stay here to try and help the club finish off the season strongly.
"I've loved it ever since I came up and I've progressed so much in a short space of time. I've also made some good friends here and would like to thank all the Oldham fans who have sent me messages of support even though I've not been able to see you in person, it does help and gives me huge confidence."
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