O'Halloran, 22, signed for the Bolton as a teenager but failed to make the leap, featuring just twice from the bench.
The Glasgow-born forward had been farmed out to Sheffield United, Carlisle and Tranmere in recent times, with his spell on the Wirral landing him three goals in 25 appearances.
O'Halloran has signed a deal until 2015 and goes into the squad for Saturday's trip to play Ross County.
A spokesman for St Johnstone said:
"Tommy Wright has been an admirer of O'Halloran for some time and once the player had secured an early release from Bolton, a deal was put together to bring him to Perth"
This deal marks the second departure from Bolton Wanderers of the day, following on from the decision not to turn Jan Gregus' loan deal into a permanent transfer.