O'Brien completed his move from Bolton Wanderers on Saturday.
Despite holding talks with their promotion rivals, O'Brien was eager to make his loan deal at Elland Road permanent.
It all hinged on an entertaining and unforgettable meeting between Leeds chief Ken Bates, O'Brien and his father.
The centre-back said: "I don't have an agent. My dad came in with me initially and he is a big bloke of around 20 stone.
"The chairman's first words were, 'We'll sign the fat one instead!' My dad replied he had been on a diet for six weeks, to which the chairman said, 'Well that isn't effing working, is it?'.
"Dad is quite quick-witted and said back, 'Well I don't know what your excuse is'. It went from there.
"There was a bit of to-ing and fro-ing and I said, 'If you offer me what Cardiff have offered then I will sign'."