The new operation, to be called Isobel, has been founded by Paul Houlding, 41, BHO's former group business development director, Steve Hastings, 45, its ex-planning chief, and the joint creative directors Rob Fletcher, 34, and Dave Alexander, 38.
It opens with a blank client list, but its founders say they are hopeful of being able to announce their first business within the next few weeks.
Their agency is contractually barred from soliciting any BHO accounts for six months, but they say they are uninterested in pursuing their former employer's business.
Houlding said: "Our priority is to keep what we do as a simple process. Agencies have become dominated by process and have lost sight of the value they add. Account handling departments are being dwarfed by finance departments counting the money the agency hasn't got."
The partners are equal shareholders in the venture, which is not being supported by outside investment.
The start-up has been christened Isobel because three of the partners have daughters who have it as one of their names. "We didn't want our names above the door because the agency's title would have been a mouthful," Houlding added. "We opted for something that's simple and memorable."
"We all have different reasons for doing it," Houlding, a former Saatchi & Saatchi staffer, said. "But we've all reached a point where we want to do something for ourselves and we felt BHO lost a bit of life after its sale to FCB. Clemmow Hornby Inge has proved there's plenty of business for start-ups that get it right."