OnePlus has begun sending out the stable Pie update to its last-gen flagships, the OnePlus 5 and OnePlus 5T. The devices were launched on Android Nougat, and this will likely be the last major update they receive.
https://www.notebookcheck.net/The-OnePlus-5-and-OnePlus-5T-receive-their-stable-Pie-updates.387341.0.html