The 18-year-old striker sealed a move from Peterborough to Boundary Park over the weekend for an undisclosed fee having impressed while on loan with Latics' north-west neighbours Bury towards the end of this term.
"I am buzzing and it is hard to put into words what it means to me to be joining Latics," Clarke-Harris told Oldham's official website.
"I am looking forward to playing regularly and also for helping the team to do well.
"I am very impressed with (manager) Lee Johnson and look forward to working with him. Hopefully it will be a good partnership.
"My aims are to play regularly, score goals and enjoy my football. If you are not happy, you don't play well and be yourself.
"I will be working hard and hopefully I will play a lot of games and score between 10 and 15 goals."