Centre-half Harrison and midfielder/defender Oliver, both 20, will go straight into Colin Cooper's squad for Saturday's home league game against Cheltenham.
Middlesbrough-born Harrison joined Sunderland's academy from Darlington in 2012 and was an unused substitute at Tottenham on the final day last season. He had a spell on loan at Bury earlier in the current campaign.
Oliver is in his second year as a professional at Sunderland after progressing through their academy and has been a regular in the Black Cats' development side.
Meanwhile, Manchester United have agreed to extend teenager Jack Barmby's loan spell with the club until the end of the season.
Barmby, son of former Tottenham and Liverpool striker Nick, joined Pools at the end of January and has made 11 appearances, scoring three goals.