He will join The Westpac Group early next year, and be responsible for media relations, government relations, internal communications, sustainability and community involvement.
O’ Riordain joined Royal Mail in 2010, replacing Mary Fagan.
Before this he had the top comms job at Lloyd’s Banking Group, and has also worked for the Irish Diplomatic Service in Dublin and Washington.
He will report to Westpac's group executive, human resources and corporate affairs, Christine Parker.
Westpac has 12.2 million customers, with branches throughout Australia, New Zealand and the near Pacific region.