It’s a basic security update, bumping the security patch to October 2019. It also updates the Google Mobile Services and tops it all off with some bug fixes and improvements. The update is rolling out in stages and it may take a few days to reach your phone.
The OnePlus 3 launched in June 2016, followed in November by the 3T. OS updates for them should have ended in November 2018, but the phones the two phones (originally running Android 6.0 Marshmallow) got Android 9 Pie in May this year. That made it three major OS updates, more than most Android phones ever get.