The Baku-Tbilisi-Ceyhan pipeline links Azerbaijan, Georgia and Turkey. Oil is then shipped to western markets.
The US-backed project was conceived 10 years ago to diversify the West's oil supplies and bypass Russia.
Oil began flowing in May. At full capacity the pipeline will carry a million barrels of crude oil a day.
A second pipeline carrying Caspian gas along the same route is expected to start working by the end of the year.
The official opening ceremony is due to take place in Ceyhan shortly.
It will be attended by the presidents of Turkey, Azerbaijan and Georgia.
This is just a fraction of rising world demand but the pipeline has been heavily backed by Washington from the very start, the BBC's Sarah Rainsford reports from Ceyhan.
The US is keen to challenge Russia's dominance of energy supply routes and to promote the Caspian as a secure additional source of fuel channelled via America's regional allies, our correspondent says. For Ankara too this is a strategic project as much as an economic one, she adds.
Ankara is currently negotiating to host several other international gas and oil pipelines, hoping to become a major transit and terminal country for fuel.Güncelleme Tarihi: 20 Eylül 2018, 18:16