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 | Sheffield UnitedSheff Utd | 21 | 14 | 5 | 2 | 30 | 11 | 19 | 45 |
| 2 | Leeds UnitedLeeds | 21 | 12 | 6 | 3 | 37 | 15 | 22 | 42 |
| 3 | Sunderland | 21 | 11 | 7 | 3 | 32 | 17 | 15 | 40 |
| 4 | Burnley | 20 | 10 | 8 | 2 | 24 | 7 | 17 | 38 |
| 5 | Blackburn RoversBlackburn | 20 | 11 | 4 | 5 | 25 | 17 | 8 | 37 |
| 6 | Middlesbrough | 21 | 10 | 4 | 7 | 35 | 25 | 10 | 34 |
| 7 | West Bromwich AlbionWest Brom | 20 | 7 | 11 | 2 | 23 | 14 | 9 | 32 |
| 8 | Watford | 19 | 9 | 4 | 6 | 27 | 25 | 2 | 31 |
| 9 | Sheffield WednesdaySheff Weds | 21 | 8 | 5 | 8 | 26 | 30 | -4 | 29 |
| 10 | Swansea CitySwansea | 21 | 7 | 6 | 8 | 23 | 22 | 1 | 27 |
| 11 | Bristol City | 21 | 6 | 9 | 6 | 26 | 26 | 0 | 27 |
| 12 | Norwich CityNorwich | 20 | 6 | 8 | 6 | 35 | 30 | 5 | 26 |
| 13 | Millwall | 20 | 6 | 7 | 7 | 20 | 18 | 2 | 25 |
| 14 | Derby CountyDerby | 21 | 6 | 6 | 9 | 26 | 26 | 0 | 24 |
| 15 | Coventry CityCoventry | 21 | 6 | 6 | 9 | 27 | 30 | -3 | 24 |
| 16 | Preston North EndPreston | 21 | 4 | 11 | 6 | 21 | 27 | -6 | 23 |
| 17 | Stoke CityStoke | 21 | 5 | 7 | 9 | 23 | 28 | -5 | 22 |
| 18 | Queens Park RangersQPR | 21 | 4 | 10 | 7 | 21 | 27 | -6 | 22 |
| 19 | Luton TownLuton | 21 | 6 | 4 | 11 | 23 | 37 | -14 | 22 |
| 20 | Oxford UnitedOxford Utd | 20 | 4 | 6 | 10 | 21 | 33 | -12 | 18 |
| 21 | Cardiff CityCardiff | 20 | 4 | 6 | 10 | 19 | 32 | -13 | 18 |
| 22 | Portsmouth | 19 | 3 | 8 | 8 | 21 | 34 | -13 | 17 |
| 23 | Plymouth ArgylePlymouth | 20 | 4 | 5 | 11 | 19 | 42 | -23 | 17 |
| 24 | Hull City | 21 | 3 | 7 | 11 | 19 | 30 | -11 | 16 |
| > Championship Full Table | |||||||||
