The purchase of the five 787-9 long-haul aircraft by the Beijing-based airline was signed off in the United States on Wednesday and announced on Thursday via the companies' official accounts on Chinese microblogging site, Weibo.
Okay Airways currently has a fleet of 26 single-corridor Boeing 737 planes, which fly to 70 destinations in China and elsewhere in Asia.
The Chinese company had signed a preliminary agreement to buy the planes back in June, when it has also made a firm order for 15 medium-sized 737 MAX planes at a catalogue price of USD 1.8 billion.
He added that the order would strengthen the airline's plans to expand into the long-haul market.
Chinese companies are on the whole profiting from the country's spectacular rate of aviation expansion, fuelled by a growing middle class and a rapidly developing tourism industry.
But competition on international routes is becoming more intense, with China's three airlines -- Air China, China Southern and China Eastern -- fighting it out to lead the sector.