The agency is believed to have been awarded the business after a final three-way shoot-out that also involved Publicis and Euro RSCG.
The news follows the appointment of PHD North to handle Otto's media account, following a five-way pitch that resulted in the account being consolidated in December.
Otto UK is understood to be launching an upmarket catalogue brand, which both Golley Slater and PHD North will be responsible for promoting. The media remit is currently thought to include TV, print and PR elements.
Otto is the fourth-biggest mail-order business in the UK, behind Littlewoods, JD Williams and Next. A subsidiary company of the huge German company Otto Group, the UK division includes the Kaleidoscope, Grattan and Freemans brands.
The company was bought from its founder by a management buy-in team led by Chris Lovell three years ago. Last month, the agency announced plans to move to London, with a view to increasing its national profile.
Previously, it was based in Cardiff and didn't have a central London presence. The win is thought to be the agency's first since its move.
Otto Group operates more than 120 companies and employs 55,000 staff in 19 countries. Last year, it hired the digital agency Cheeze, which is part of Digital Marketing Group, to create a marketing strategy for Duck.co.uk, its online electrical goods site.
The UK subsidiary said that sales over the Christmas period rose by 14 per cent.
Golley Slater and Otto UK refused to comment.