Ocean, which specialises in large-scale advertising sites across the UK, such as the Imax cinema and Centre Point in London, was founded two years ago.
Chief executive Damian Cox said additional capital was now required. He added: "There is a big opportunity in what is now a deflationary outdoor market.
"We can now go out and do deals with landlords of sites on better terms. This capital is expansion capital and it lets us go out and develop existing sites and target new areas."
Johnny Hewett, chief executive of Smedvig Capital, claimed the outdoor advertising market has "huge potential".
He said: "Outdoor already possesses a highly attractive portfolio of premium sites and we look forward to working with the team as it further expands the existing strong pipeline of new locations."
Smedvig Capital is a London-based private equity firm that has invested in more than 30 businesses since its founding in 1996. Current investments include hire- car operator Streetcar; Lovejuice, the juice-bar chain; and international cake-maker, Almondy.