Wood, who joins the agency from Rapier, and O'Sullivan, who joins from Targetbase Claydon Heeley, have made the move off the back of a number of recent wins for Stephen Francis Whitson, including RSA and QVC.
The pair will be responsible for increasing the behavioural and attitudinal insights across the agency's clients.
While working at Rapier, Wood worked on the agency's Lloyds TSB business. Before that, she worked at Archibald Ingall Stretton on the O2 account.
O'Sullivan started her career at Claydon Heeley, before spending time at agencies including Beattie McGuinness Bungay and R/GA.
Mel Stanley, the head of account planning at Stephen Francis Whitson, said: "Helena and Isabelle represent some of the brightest young planning talent in the business. They both have experience across the whole range of communications and like every young planner now are digital natives, utterly comfortable in the online environment."
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