The 27-year-old joins former Town team-mate Andy Holdsworth in making the trip down the M62 to Boundary Park this summer.
Parker began his career with St Johnstone before moving to Blackpool where he spent four years. He joined Huddersfield in May 2008 and had a spell on loan at Hartlepool last season.
"I am delighted to have got Keigan," Oldham manager Dave Penney told his club's official website.
"He is an exciting player with the sort of pace that unsettles defenders and he knows what our league is all about."