Oil has flowed from the largest development project in the North Sea for a decade.
More than 300 million barrels of heavy crude oil are expected to be extracted from the Mariner Field located 95 miles east of Shetland.
Norwegian state-owned Equinor said the field was a "cornerstone" in its expansion into British waters.
But the environmental organization Friends of the Earth Scotland insisted that the oil should be left in the ground.
The field was first discovered in 1981 but taken over in 2007 by Equinor and then by Statoil.
Low flow rates meant that recovery of the oil was only economically viable 25 years later.
Hedda Felin, Senior Vice President at Equinor, said: "The Mariner Field is entering into an important milestone as it is our first UK company and Mariner is our cornerstone, contributing to our commitment to being a safer and more reliable one temporary energy partner for Great Britain.
"We have a lot of new technologies that need to be applied to the field, and we were able to get much better data about the ground so we could increase the recoverable volumes.
"Carbon and climate are central to everything we do, and we want to drive the energy transition in the right way.
"We believe that both oil and gas and renewable energy are needed, there are different opinions on how fast it will go, but we will be part of that path."
Production drilling began in December 2016 and over £ 6 billion has been spent on the project over the last seven years.
This has created 800 jobs and another 700 permanent jobs.
At peak times, around 70,000 barrels per day are expected to be produced.
Mike Tholen of the British industry association Oil and Gas said: "Mariner's first oil is a significant achievement for Equinor and its groundbreaking development in this area, which makes the company a major player in the British North Sea sector will have long lasting positive effects on the industry Region.
"As the largest offshore development in the UK for a decade, Mariner has used breakthrough technologies to put into service a field that was first discovered almost 40 years ago, resulting in thousands of highly skilled jobs throughout the development and in existence Country created. " will support many more in the coming years.
Mariner is one of the largest development projects in the oil and gas sector of the North Sea for ten years.
Production has begun to cover up to 5% of UK gas demand just months after the start of the giant Culzean field.
Mary Church of Friends of the Earth, Scotland, said: "The Mariner project is a serious bad news for life on Earth, and we know that we need to stop burning oil and gas if we want to avoid climate change, So every new area like this takes us in the wrong direction.
"It is time for the Scottish and British governments to stop oil and gas expansion and instead reorient support and subsidies to create decent jobs in the economy for renewable energy."