Operator Repsol Sinopec Resources UK said first oil had been achieved at the Shaw field, about 200km (124 miles) east of Aberdeen.
Shaw is part of the Montrose Area Redevelopment (MAR).
Repsol Sinopec said MAR could extend the life of a number of fields in the area to beyond 2030.
The project has involved modifying and expanding existing Montrose Area infrastructure, including developing three new fields - Godwin, Cayley and Shaw.
A new bridge-linked platform (BLP), connected to Montrose Alpha, will provide additional process and plant support facilities for the project.
First oil is expected from Cayley by the end of the second quarter, with gross production expected to peak at up to 40,000 barrels a day.
Repsol Sinopec managing director Bill Dunnett said the Montrose Area was a "key part" of the company's long-term future.
He added: "This is a major achievement for this business and will help to consolidate the transformation that Repsol Sinopec Resources has undergone in the last two years."