The team joins after a seven-year stint at RKCR/Y&R, which they joined as graduates in 1997.
The team's credits include campaigns for Ford, Pirelli, Virgin Atlantic, the Energy Saving Trust and Vehicle Crime Reduction for COI Communications.
Their "biggest for sport" work for The Times won awards including two silvers and two commendations at the Campaign Poster Awards earlier this month. A cinema commercial for the newspaper, "director", won a bronze at the British Television Advertising Awards in March.
McGrann and Clough are the latest in a number of creative appointments O'Keeffe has made.
In July, Ewan Paterson, the former joint creative director of DDB, joined BBH as its creative director and, in August, O'Keeffe hired the Fallon team Dave Masterman and Ed Edwards.
In March, the agency hired Marc Shillum as the creative director on its flagship Levi's account.