The Chinese variants of OnePlus phones, which run Hydrogen OS rather than Oxygen OS, are typically first to see new updates and Android versions. The latest Hydrogen OS Open Beta is based on Pie. Therefore, OnePlus 6 units in other countries may have access to a similar build soon.
https://www.notebookcheck.net/OnePlus-6-New-Chinese-variant-ROM-release-suggests-official-Pie-on-the-way-soon.323488.0.html