When Android Pie was launched, OnePlus said that they'll update all their phones launched after the OnePlus 3 to Android Pie. The list includes OnePlus 3, 3T, 5, 5T, 6. All these devices have already received the update, except the OnePlus 3 and 3T. People were waiting for the update for a long time now and it looks like OnePlus has finally decided to roll out the updates.
Strangely, the Pie beta in China was launched back in July last year. OnePlus took around a year to release it for their Global devices.
How to update:
You'll need to sign-up for the update from here. The registration is closing on March 21 at 2 PM GMT. Do note that you'll need some experience in flashing ROMs, or you can watch a Youtube video or tutorial if you don't want to brick your phone. An account on OnePlus Forum is also required as you'll be asked to enter your profile's link. After successfully updating it, you'll have to test several bugs and report them to OnePlus.
If you are the one using a OnePlus 3 or 3T as a daily driver, we suggest you not to update and wait for a stable release. As said, the beta update will come with several bugs, which might make your phone unusable. Though OnePlus will solve the bugs as soon as possible and release a more stable update very often, still, you'll have to deal with them.