Bottle won the account following a competitive seven-way pitch. The retained account was previously held by McCann.
Bottle's remit will include helping its client to offer businesses advice on issues including Brexit, productivity and access to finance.
Lynn Morrison, head of business engagement at Opus Energy, said: "We are ready to drive forward our external communications across a range of audiences and media. Bottle has proven experience in the SME, corporate trade and national media and this results-based success is something we wish to replicate in our communications."
Holly Tyzack, head of editorial at Bottle, who will lead the account, added: "Opus Energy is a leading name in the energy sector and we are delighted to be working with such an informed and developed brand. Its owned channels, such as the brilliant Opus Energy Gift website, gives us an established platform to build on and comment on pressing issues affecting businesses today.
"The team will use its energy sector knowledge combined with its SME and business expertise to engage Opus Energy in debates affecting UK businesses and enhance its already stellar reputation by showing it to be a leading thought leader in its field."