OnePlus's flagships from 2018 — the 6 and the 6T — are receiving an OxygenOS update, but it's not the one you were probably hoping for. It doesn't bring Android 11 to the phones, which shouldn't be very surprising since the 7 and the 7T series are yet to get that update, too.
The OxygenOS 10.3.9 build comes with the February security patch and updates the GMS (Google Mobile Services) package to 2020.12. While OnePlus isn't delivering these releases as quickly as Samsung, it's keeping its promise of publishing bi-monthly updates. The changelog also mentions that some known issues have been fixed, but it doesn't shed light on what has been addressed in particular.
Since this is a staged rollout, you may not receive the update immediately. It's also worth noting that using a VPN might not help since the rollout isn't region-based.
Given that OnePlus usually delivers at least two Android updates to its flagships, 6T owners should expect to receive Android 11. This probably can't be said for the 6, though, which has already received two major updates. Still, until we hear it from the horse's mouth, keep your hopes high — OnePlus phones are usually updated in pairs.