Both O'Farrell and Soul deny that his departure is the result of a rift between the two parties, with O'Farrell claiming that he remains on friendly terms with his former partners. "They are my mates," he insisted.
O'Farrell has not been at the agency for the past two months and it is not known if his severance package has been agreed.
Explaining his reason for leaving, O'Farrell said: "I've always had the ambition to be partner of a creative ad business and having done that successfully for three years we've looked at the situation and realised I'd be happier moving on."
Soul was set up three years ago by a team of directors from Bartle Bogle Hegarty.
O'Farrell, a group business director on the Whitbread account, joined BBH's executive creative director, Bruce Crouch, its head of design, Andy Bird, and the One2One group business director, Duncan Bird, as partners in the start-up.
They were joined by Kevin Brown, a media director at BBH's erstwhile media partner Starcom Motive, to make up the fifth member of the Soul quintet.
Before joining BBH, O'Farrell worked at Abbott Mead Vickers BBDO on the BT account.