GroupM CEO Aimee Buchanan has raided her old stomping ground for Seb Rennie's replacement, hiring OMD's Melissa Hey as Chief Investment Officer. Rennie is leaving for Southern Cross Austereo.

Hey, who joins GroupM's executive committee, makes the switch after 12 years with OMD, six heading investments for Australia's biggest ad buying agency. She has also held senior roles at Starcom and started out at Carat in a career spanning 23 years.

"We're thrilled to bring Melissa's expertise, gravitas and capability into the business to lead our Responsible Investment approach and help us shape the next era of media," said Buchanan. "She will bring immense value."

Hey thanked OMD but said the opportunity to take on a new challenge was "unmissable".