The 22-year-old joined on non-contract terms earlier this month, but has sufficiently impressed to gain a permanent contract with the Latics.
"Danny has done really well since he arrived. He is technically very good and an excellent finisher," Oldham boss Lee Johnson told the club's official website.
"He needs to become a man in his personality on the pitch and, if we can get that into his game, he can become a top player and back to where he was as a youngster.
"Danny provides us with another attacking option - we have lots of them - and I believe this side will score goals whoever plays up front."
Philliskirk began his youth career at Chelsea.