Jane Cunningham joins as the head of planning, overseeing a team of up to 40 planners from late October.
She reports to the Ogilvy group planning director, Mark Earls. Currently the managing director of Tribal DDB, Cunningham has also worked as a senior planner at BMP DDB after starting her career at Grey.
Earls said he has wanted to "up the ante" in the department since his arrival at the agency last year, when he broke up the existing structure and made one large department. As well as the agency team of 40, he has around 30 other consultant planners working on other parts of agency business.
He said: "I've admired Jane's work for a while - it's a small world and you hear about people who are good. She is someone who's smart and thinks big, and has lots of experience of advertising planning. She's also well-versed in digital media from her time at Tribal."
Cunningham added: "My heart's always been in planning, and it's a shame to leave Tribal. But O&M has a really strong advertising and interactive legacy, which is really attractive."
Cunningham will be joined by three new planning partners, each taking responsibility for one of O&M's key accounts. Christine Asbury is the vice-chair of the Account Planning Group and has worked previously at Colman RSCG, Still Price and was the planning director at Griffin Bacal. She will work on Lever Faberge's Comfort account.
Jane Almey joins from Saatchi & Saatchi, where she was a senior planner working on the NSPCC and Head & Shoulders accounts, and previously worked in research. She will work on the Dove account with Tracey Rainey, who joins from Rainey Kelly Campbell Roalfe/Y&R.