The 20-year-old attacker had fallen out of the first-team picture under Johnson's predecessor Paul Dickov but capped his resurgence with a first goal for the club in Saturday's final day draw at Leyton Orient.
Millar told Oldham's official website: "I am buzzing to have scored my first goal for Latics.
"I have worked hard to get into the team and Lee Johnson has come in and given me a chance.
"He has been a big help and also put me up front where I prefer playing.
"I have played as a winger with Latics, but before I came here I was always a striker.
"Lee Johnson must have seen it in me that I can play up front."
Johnson will dust off his boots to turn out for Oldham's second string in Monday night's Manchester Senior Cup clash with Manchester City at Boundary Park.
The team will feature four trialists, including former Latics full-back Paul Black and winger Matthew Whichelow.
Whichelow was released by Watford in February having completed a loan spell at Accrington earlier this season, where he made four appearances.
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