He scored seven tries in 14 appearances from centre, before switching to full-back last season, scoring 15 tries in as many appearances.
Coach Joey Grima said: “Sean is a superb athlete and has scored tries for fun with Oxford. He will be a great addition to our squad.
“We are pleased to help give players from clubs in the region an opportunity to perform at a higher level. Oxford have been fantastic to deal with since we first approached them about Morris and we’d like to thank them for their professionalism.”
Morris played two games for the Broncos on trial last season.
He said: “I’m really pleased to be back. I was a little disappointed at the end of the trial last year. I spent the year working hard and putting in the effort down at Oxford with the aim of giving myself another shot at going forward at the end of the season. For the opportunity to come around again is fantastic.”