Among ARM's largest outdoor clients is online gambling firm 888.com, a prolific poster advertiser which accounts for much of the group's spend in the sector.
The win is a coup for Open, which will have billings of £10m by the end of this year, and is a significantly smaller player than the three major poster buying specialists Kinetic, Posterscope and IPM.
There was no competitive pitch for the work. ARM had been a client of Kinetic and its predecessor agency, Portland, for a number of years, and Open had been speaking to ARM for 18 months before the media agency decided to make the switch. ARM has total billings of £71m a year, but spends around £3m in outdoor. It specialises in direct response campaigns.
Open founder and chief executive Andrew Kelly said: "They still see themselves as a young developing company and felt Open was a better cultural fit. We were able to demonstrate an understanding of their clients and offer more innovative solutions. We'll be an incubator into the outdoor market for clients that wouldn't normally fit into outdoor."
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