Mason will take up the position as the ODA's most senior architect in August, ending her six-year career with architectural watchdog CABE.
Richard Simmons, CABE chief executive, said: 'Selina has made a brilliant contribution to CABE's work, first as head of enabling and latterly as director of architecture and design review.
'She has sustained CABE's reputation as the national centre of excellence for design review and taken us into new areas to keep it effective and relevant. We will all miss her.
'CABE has been working hard since the ODA was set up to ensure it delivers a well-designed Olympics. Selina's move is, in many ways, symbolic of our joint commitment to excellent design. I wish her well in her new role,' he added.
CABE said it would now begin the process of recruiting Mason's replacement.