The 22-year-old made just two appearances for the Trotters last season.
"I'm delighted to have Joe here," Oxford head coach Michael Appleton told BBC Sport. "He is six foot tall, he has pace and he uses the ball well."
Riley becomes Oxford's fourth signing of the summer following the arrivals of Danny Hylton, Michael Collins and George Long.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sunderland | 15 | 9 | 4 | 2 | 25 | 11 | 14 | 31 |
| 2 | Sheffield UnitedSheff Utd | 15 | 10 | 3 | 2 | 19 | 7 | 12 | 31 |
| 3 | Leeds UnitedLeeds | 15 | 8 | 5 | 2 | 24 | 9 | 15 | 29 |
| 4 | Burnley | 15 | 7 | 6 | 2 | 18 | 6 | 12 | 27 |
| 5 | West Bromwich AlbionWest Brom | 15 | 6 | 7 | 2 | 16 | 9 | 7 | 25 |
| 6 | Watford | 15 | 8 | 1 | 6 | 23 | 22 | 1 | 25 |
| 7 | Middlesbrough | 15 | 7 | 3 | 5 | 23 | 17 | 6 | 24 |
| 8 | Millwall | 15 | 6 | 5 | 4 | 18 | 13 | 5 | 23 |
| 9 | Blackburn RoversBlackburn | 15 | 6 | 4 | 5 | 19 | 17 | 2 | 22 |
| 10 | Bristol City | 15 | 5 | 7 | 3 | 20 | 19 | 1 | 22 |
| 11 | Swansea CitySwansea | 15 | 5 | 4 | 6 | 11 | 10 | 1 | 19 |
| 12 | Derby CountyDerby | 15 | 5 | 4 | 6 | 19 | 19 | 0 | 19 |
| 13 | Stoke CityStoke | 15 | 5 | 4 | 6 | 18 | 19 | -1 | 19 |
| 14 | Norwich CityNorwich | 15 | 4 | 6 | 5 | 23 | 22 | 1 | 18 |
| 15 | Sheffield WednesdaySheff Weds | 15 | 5 | 3 | 7 | 17 | 25 | -8 | 18 |
| 16 | Oxford UnitedOxford Utd | 15 | 4 | 5 | 6 | 17 | 18 | -1 | 17 |
| 17 | Coventry CityCoventry | 15 | 4 | 4 | 7 | 20 | 21 | -1 | 16 |
| 18 | Plymouth ArgylePlymouth | 15 | 4 | 4 | 7 | 15 | 26 | -11 | 16 |
| 19 | Hull City | 15 | 3 | 6 | 6 | 16 | 20 | -4 | 15 |
| 20 | Preston North EndPreston | 15 | 3 | 6 | 6 | 15 | 23 | -8 | 15 |
| 21 | Luton TownLuton | 15 | 4 | 3 | 8 | 17 | 26 | -9 | 15 |
| 22 | Cardiff CityCardiff | 15 | 4 | 3 | 8 | 14 | 23 | -9 | 15 |
| 23 | Portsmouth | 15 | 2 | 6 | 7 | 16 | 28 | -12 | 12 |
| 24 | Queens Park RangersQPR | 15 | 1 | 7 | 7 | 12 | 25 | -13 | 10 |
| > Championship Full Table | |||||||||
