Italian firm Eni has begun producing oil from its Goliat field in the Norwegian Arctic, making it the world's most northerly oilfield in production.
"It started last night," said an Eni spokesman on Sunday.
"We have reached an important milestone not only for Eni, but also for the industry and for Norway."
The start-up of the field has been delayed several times since the initial planned date of 2012.
Costs also surged to 46.7 billion kroner ($A7.46 billion) from an original estimate of around 30 billion kroner in 2009, when the development plan was approved.
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Output from the field is due to peak at around 100,000 barrels of oil per day.
The production facility also has storage capacity for one million barrels of oil.
Asked about the multiple delays, Eni said Goliat was a "pioneering project" that involved new technology on which the oil industry would build for the future.
ENI has a 65 per cent stake in the field, with Norway's Statoil holding the remaining 35 per cent.
