The axing of former Whites winger Kewell and also his assistant Alan Maybury comes with Oldham sat 16th in the League Two table following Saturday's goalless draw at home to second-bottom Southend United.
The Latics have now gone through four managers since Paul Scholes left the club in March 2019.
A club statement released by Oldham on Sunday evening read: "Harry Kewell has today been relieved of his duties as Head Coach of Oldham Athletic.
"Arriving in August 2020 he now leaves along with his assistant, Alan Maybury.
"We would like to thank both for their efforts this season and wish them well in the future.
"An announcement on a replacement will be communicated in due course with no further comment made until then."
McCalmont joined Oldham on a season-long loan under Kewell back in September and played the full duration of Saturday's goalless draw.
The 20-year-old has quickly amassed 25 appearances for Oldham, including 18 league starts.
The Northern Ireland international has netted five times for the Latics in league and cup.
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Thank you Laura Collins