Fishburn was appointed to the retained brief following a pitch understood to have involved a number of leading agencies and described by Fishburn public affairs MD Rory Scanlan as "furiously competitive".
Opower specialises in cloud-based consumer engagement and energy efficiency services for the utility industry, designed to improve engagement with energy suppliers and help consumers cut the cost of their energy usage.
The company was founded in the US in 2007 and expanded into the UK at the end of 2011, partnering with energy companies E.On and First Utility.
The company appointed its first UK public affairs manager, Claire Jakobsson, at the end of 2013 and she oversaw the pitch process.
Last month Opower appointed US agency LaunchSquad to a brief involving elevating its brand globally and domestically.