The 22-year-old, who has represented Scotland at Under-21 level, has agreed an 18-month contract at McDiarmid Park.
O'Halloran failed to make a league appearance during his time at the Reebok Stadium, with his two matches for the club coming in cup competitions.
The Glasgow-born forward has also enjoyed loan spells at Sheffield United and Tranmere Rovers.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Birmingham CityBirmingham | 9 | 7 | 1 | 1 | 17 | 9 | 8 | 22 |
| 2 | Wrexham | 10 | 6 | 2 | 2 | 18 | 8 | 10 | 20 |
| 3 | Mansfield TownMansfield | 9 | 6 | 2 | 1 | 17 | 11 | 6 | 20 |
| 4 | Lincoln CityLincoln | 9 | 5 | 3 | 1 | 15 | 8 | 7 | 18 |
| 5 | Wycombe WanderersWycombe | 9 | 5 | 2 | 2 | 17 | 13 | 4 | 17 |
| 6 | Stockport CountyStockport | 9 | 4 | 4 | 1 | 13 | 7 | 6 | 16 |
| 7 | Exeter CityExeter | 9 | 5 | 1 | 3 | 10 | 6 | 4 | 16 |
| 8 | Charlton AthleticCharlton | 10 | 5 | 1 | 4 | 10 | 9 | 1 | 16 |
| 9 | Huddersfield TownHuddersfield | 10 | 5 | 0 | 5 | 14 | 11 | 3 | 15 |
| 10 | Blackpool | 10 | 4 | 3 | 3 | 17 | 16 | 1 | 15 |
| 11 | Barnsley | 10 | 4 | 3 | 3 | 15 | 15 | 0 | 15 |
| 12 | Stevenage | 10 | 4 | 2 | 4 | 8 | 7 | 1 | 14 |
| 13 | Bolton WanderersBolton | 9 | 4 | 2 | 3 | 15 | 15 | 0 | 14 |
| 14 | Peterborough UnitedPeterborough | 10 | 4 | 2 | 4 | 16 | 18 | -2 | 14 |
| 15 | Wigan AthleticWigan | 10 | 3 | 4 | 3 | 9 | 5 | 4 | 13 |
| 16 | Reading | 9 | 4 | 1 | 4 | 13 | 14 | -1 | 13 |
| 17 | Rotherham UnitedRotherham | 10 | 3 | 4 | 3 | 9 | 11 | -2 | 13 |
| 18 | Bristol Rovers | 10 | 4 | 1 | 5 | 13 | 16 | -3 | 13 |
| 19 | Leyton Orient | 10 | 2 | 2 | 6 | 10 | 14 | -4 | 8 |
| 20 | Northampton TownNorthampton | 9 | 2 | 2 | 5 | 11 | 17 | -6 | 8 |
| 21 | Crawley TownCrawley | 9 | 2 | 1 | 6 | 5 | 13 | -8 | 7 |
| 22 | Shrewsbury TownShrewsbury | 10 | 1 | 2 | 7 | 8 | 17 | -9 | 5 |
| 23 | Burton Albion | 9 | 0 | 4 | 5 | 10 | 19 | -9 | 4 |
| 24 | Cambridge UnitedCambridge | 9 | 0 | 1 | 8 | 6 | 17 | -11 | 1 |
| > League One Full Table | |||||||||
