A new version of OxygenOS for the OnePlus 3 and 3T is finally here. However, there are a few slight caveats: it is still in beta (albeit an official Community Beta), and thus may still have some bugs. On the other hand, it has an up-to-date security patch - and is based on Android Pie (9.0).
https://www.notebookcheck.net/OnePlus-3-3T-new-OxygenOS-beta-based-on-Android-Pie-available-now.418147.0.html