The forward has joined Oldham until the end of the season, with the Sky Bet League 1 club managed by his former Kilmarnock team-mate, Lee Johnson.
The move comes after Harkins proved unable to consolidate a starting place with St Mirren. His work ethic has since emerged as a reason for his lack of playing time.
"There were one or two little disagreements," said Harkins. "I'm delighted to get the chance at Oldham - as soon as I got the call I was itching to get here. I look forward to the challenge."