The long-serving manager has been recalled after her replacement, Michael Parker, was diagnosed with leukaemia.
Mrs Bremner joined PSO in 1993.
She retired in March last year, and planned to join her husband, Graeme, in Botswana, where he had an academic position.
However, Mr Parker's illness had resulted in Mrs Bremner standing in as interim chief executive.
Mr Parker was diagnosed with acute leukaemia just before Christmas and had been on leave since.
Board chairman David Richardson has announced he will step down from both the chairmanship and the board. ''to allow an incoming chair to provide continuity through the appointment process and transition to a new chief executive,'' he said.
His replacement is accountant Tim Mepham, who has been on the PSO board for 10 years and deputy chairman since 2013.
