The U's struck a deal on a six-month loan for the 22-year-old, who is currently contracted to Cypriot side Apollon Limassol.
O'Neill is eligible to play internationally for Ireland and the USA, having represented the latter at Under-20 and Under-23 level.
The defender started his career in the MLS with Colorado Rapids, for whom he scored one goal in 52 appearances.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Wycombe WanderersWycombe | 14 | 10 | 2 | 2 | 32 | 16 | 16 | 32 |
| 2 | Birmingham CityBirmingham | 13 | 9 | 3 | 1 | 24 | 12 | 12 | 30 |
| 3 | Wrexham | 15 | 8 | 4 | 3 | 22 | 11 | 11 | 28 |
| 4 | Stockport CountyStockport | 16 | 7 | 6 | 3 | 26 | 17 | 9 | 27 |
| 5 | Barnsley | 15 | 7 | 5 | 3 | 24 | 19 | 5 | 26 |
| 6 | Lincoln CityLincoln | 15 | 7 | 5 | 3 | 21 | 17 | 4 | 26 |
| 7 | Mansfield TownMansfield | 13 | 7 | 3 | 3 | 20 | 15 | 5 | 24 |
| 8 | Huddersfield TownHuddersfield | 14 | 7 | 2 | 5 | 21 | 14 | 7 | 23 |
| 9 | Exeter CityExeter | 14 | 7 | 2 | 5 | 14 | 10 | 4 | 23 |
| 10 | Reading | 14 | 7 | 2 | 5 | 22 | 21 | 1 | 23 |
| 11 | Bolton WanderersBolton | 14 | 7 | 2 | 5 | 22 | 24 | -2 | 23 |
| 12 | Peterborough UnitedPeterborough | 15 | 6 | 3 | 6 | 31 | 27 | 4 | 21 |
| 13 | Charlton AthleticCharlton | 14 | 5 | 4 | 5 | 15 | 15 | 0 | 19 |
| 14 | Bristol Rovers | 15 | 5 | 3 | 7 | 16 | 21 | -5 | 18 |
| 15 | Stevenage | 15 | 5 | 3 | 7 | 11 | 16 | -5 | 18 |
| 16 | Northampton TownNorthampton | 15 | 4 | 5 | 6 | 18 | 21 | -3 | 17 |
| 17 | Rotherham UnitedRotherham | 15 | 4 | 5 | 6 | 14 | 18 | -4 | 17 |
| 18 | Blackpool | 15 | 4 | 5 | 6 | 21 | 28 | -7 | 17 |
| 19 | Wigan AthleticWigan | 14 | 3 | 5 | 6 | 12 | 12 | 0 | 14 |
| 20 | Leyton Orient | 14 | 4 | 2 | 8 | 14 | 18 | -4 | 14 |
| 21 | Crawley TownCrawley | 16 | 3 | 4 | 9 | 14 | 27 | -13 | 13 |
| 22 | Cambridge UnitedCambridge | 14 | 3 | 2 | 9 | 13 | 24 | -11 | 11 |
| 23 | Burton Albion | 14 | 1 | 5 | 8 | 15 | 25 | -10 | 8 |
| 24 | Shrewsbury TownShrewsbury | 15 | 2 | 2 | 11 | 13 | 27 | -14 | 8 |
| > League One Full Table | |||||||||
