The Norwegian search engine said it chose Fortress because of its use of renewable energy, its low Power Usage Effectiveness (PUE) and its high availability of fiber.
Opera Program manager for hosting Trygve Jarholt said Opera is increasingly looking to green data center operations, much like search giant Google, as its operations grow.
"Going green means going in the right direction," Jarholt said.
"Opera is more than doubling the traffic each year, and we need partners who can support our requirements as a fast growing, data center intensive company, as well as keeping Mother Nature happy."
Fortress Colocation Centers use 2N cooling and offers metered power. It has facilities in the West cost of the US, including in Washington and Seattle.
Opera also signed up with Thor data center in Iceland last year, which boasts the use of 100% renewable energy.
And in 2009 it announced it had leased space in Poland for its Opera Mini and Opera Turbo browsers, designed for mobile devices.