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 the 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 on 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 they felt Open was a better cultural fit. We'll be an incubator into the outdoor market for clients that wouldn't normally fit into outdoor."
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