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 | Accrington StanleyAccrington | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 2 | AFC Wimbledon | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 3 | Bolton WanderersBolton | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 4 | Burton Albion | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 5 | Cambridge UnitedCambridge | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 6 | Charlton AthleticCharlton | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 7 | Cheltenham TownCheltenham | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 8 | Crewe AlexandraCrewe | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 9 | Doncaster RoversDoncaster | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 10 | Fleetwood TownFleetwood | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 11 | Gillingham | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 12 | Ipswich TownIpswich | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 13 | Lincoln CityLincoln | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 14 | MK Dons | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 15 | Morecambe | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 16 | Oxford UnitedOxford Utd | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 17 | Plymouth ArgylePlymouth | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 18 | Portsmouth | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 19 | Rotherham UnitedRotherham | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 20 | Sheffield WednesdaySheff Weds | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 21 | Shrewsbury TownShrewsbury | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 22 | Sunderland | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 23 | Wigan AthleticWigan | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 24 | Wycombe WanderersWycombe | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
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