[2] But the offer to become a professional footballer was a tough decision to make for O'Callaghan as he had been accepted into Loughborough College to undertake a physical education teaching diploma.
[2] After six months in Donny's reserves he was promoted to the first team and he developed a fine partnership with Peter Kitchen and he top scored in 1975–76 with 28 goals.
By the time his goal tally had reached 77 for Doncaster he had attracted the attentions of bigger clubs and on 1 March 1978 Stoke City manager Alan Durban signed him for £40,000.
[2] With Stoke making a slow start in 1980–81 Durban moved O'Callaghan, who had lost his place to the younger Lee Chapman, to centre back filling in for the injured Denis Smith.
[2] Tall at 6 feet 2 inches (188 cm) and weighing over 13 stone (180 lb; 83 kg), 'Big Bren', as he was known, worked well as a willing target man, shielding the ball and acting as a link between midfield and a pacey striker.