PHD North picked up the two accounts, which have combined billings of £1.5 million, without a pitch.
The Orangina account was previously handled by Star-com Motive. The Strathmore brand is a new acquisition for the soft drinks company and PHD North will be responsible for the launch campaign.
The move consolidates all of AG Barr's drinks portfolio into the agency, which holds Irn-Bru, Diet Irn-Bru, Irn-Bru 32, Tizer and St Clements.
The AG Barr account was previously handled by PHD London, and moved to PHD North last year. Leith and BDH TBWA handle creative for the accounts.
Adrian Troy, the head of marketing at AG Barr, said: "The agency has always delivered effective media solutions across our brand portfolio."
Jason Spencer, the managing director of PHD North, said: "The soft drinks market is at an interesting point with changing consumer habits and new legislation forcing an inevitable shake-up in the status quo. PHD North hopes to pioneer fresh media thinking to make Orangina and Strathmore successful in 2007."