The 22-year-old former Leicester City forward was wanted by several clubs but had no hesitation in choosing to join the Latics.
"When Rotherham went into administration the opportunity came along to leave because we weren't being paid, and I've known of Oldham's interest since then," he said.
"I could have gone to another League One club but that move wasn't right for me or my family - I was holding out for Oldham.
"I'm glad to be able to move up a level and join a big club like Oldham. Coming in and seeing the set up is just making me more and more excited about it."