OnePlus has started a closed beta program for the upcoming Android Pie update for the OnePlus 3 and 3T.
This way, they can test the software before it gets released to the public.
In the last few days, OnePlus has selected a small group of users to install these betas. The application form has already been closed, but see the official announcement here.
It's expcected that a public (beta) release follows soon.
Note: the closed betas are not offered in Oxygen Updater, as this is against the NDA set in place by OnePlus.