The 30-year-old former Wallabies wing won 86 caps and helped Australia land the world title in 1999 with victory over France.
He also went on as a replacement during the 2003 final when Australia lost to England in Sydney.
Roff will be available for the Varsity Match between Oxford and Cambridge on December 12. He shortly begins studying at Oxford.
"We are obviously thrilled that Joe is coming to study at Oxford and we look forward to him playing at the club," said Oxford University rugby administrator Tim Stevens.
"His experience and talent speaks for itself, and everyone is excited about his involvement not only with the Varsity Match, but the whole season."