TransCanada has begun injecting crude oil into its $2.3 billion pipeline between Cushing and the Gulf Coast.
The company says that Saturday's development means the 485-mile pipeline is another step closer to commercial operation.
The Oklahoman reports the pipeline, called the Gulf Coast Project, is the southern leg of TransCanada's proposed Keystone XL pipeline, which would transport oil from Canada's oil sands and the Bakken Shale in North Dakota to refineries in the Houston area.
President Barack Obama refused to issue a permit for part of the project last year amid concerns about its potential impact on a large aquifer in Nebraska. The administration is considering another application, but TransCanada has received clearance for the pipeline's southern leg.
It will carry up to 700,000 barrels of oil a day.