Users in China have reportedly started receiving the beta version of HydrogenOS based on Android 9 Pie. This comes after several months of internal testing of the Chinese-exclusive ROM indicating that the final version is not far off. OnePlus is yet to announce the roll out of OxygenOS beta based on Android Pie but given that the company has recently invited testers for a closed beta, global roll out can be expected soon.
https://www.notebookcheck.net/OnePlus-3-and-3T-reportedly-getting-new-HydrogenOS-beta-update-based-on-Android-9-Pie.415198.0.html