Oil giant BP has agreed to sell its interest in a number of North Sea assets in a deal worth about £300m.
Serica Energy has bought into the Bruce, Keith and Rhum fields.
BP said it remained committed to the North Sea, and would continue to invest.
Serica said the deal would make it one of the leading British independent oil companies. Subject to approvals, the sale is expected to be finalised in the third quarter of 2018.
Bernard Looney of BP said: "This is an example of BP's upstream strategy in action - refreshing our portfolio and focusing our activity on assets which will add most value over the long-term.
"We remain committed to the North Sea and continue to invest. We expect our production there to double to around 200,000 barrels equivalent a day by 2020 through new projects like Quad 204 and Clair Ridge.
"While the Bruce assets are no longer core to BP, we are confident that Serica is the right owner and operator to maximise their continuing value for both companies and for the UK."
Serica Energy chief executive officer Mitch Flegg told BBC Scotland: "We have been chasing this opportunity for a number of months.
"It really catapults Serica into a different league."
He said the company would set up an office in Aberdeen and recruit more staff.