The right-shot defenceman was selected by the New York Rangers in the fourth-round (104th-overall) of the 2014 NHL Draft.
This year with the IceDogs, the 20-year-old has 14 goals, 37 assists and 51 points in 58 games. The 6-foot-5, 225-pound blueliner also has eight power play goals, 16 power play points and serves as captain of the team.
"Ryan is a big right-shot d man that's a little bit of a late bloomer," said Oilers President of Hockey Operations and General Manager Peter Chiarelli. "We scouted him, he's really had a strong year and he's a good prospect. We're happy he signed with us. He liked what we did with our young guys this year."
Mantha has played 175 career OHL games with the IceDogs, recording 101 points (29G, 72A) and 163 penalty minutes.
Chiarelli mentioned there's a good chance Mantha sees time in Bakersfield with the Condors.
"He'll be playing in Bakersfield probably at the end of the year."
