The 20-year-old full-back, who made 10 appearances for the U’s last season, will now stay with the Kassam Stadium until at least 2017.
Long told the club’s official website: “It was a straightforward decision. It’s been a long summer trying to sort it out but I’m happy now.
“I’m going to come back and work as hard as I can to give myself the best possible chance to get in the starting line-up.
“I’ve kept myself in good shape over the summer and can’t wait to get back into pre-season.”
Long made his U’s debut at Accrington in April 2013 and scored his first goal for the club against Southend in January.