HydrogenOS is a Chinese ROM by OnePlus that runs on almost all of their devices. The main difference between HydrogenOS and OxygenOS is that HydrogenOS doesn’t come with Google services, such as the Play Store and others. But, it has a history of receiving updates earlier than OxygenOS. Just now, it received Android Pie-based 2nd beta for OnePlus 6, which brings September security patches.
As you can see, the update contains a bunch of bug fixes, along with the September security patches. The first beta version was already more stable than the Android P Developer Preview builds, so the second beta should be better as it’s more optimized. It also contains OnePlus’ proprietary features and tweaks.
If you’re already running HydrogenOS Beta 1, then you should have already received the OTA update. If you’re running OxygenOS or any other custom ROM and want to try Google-less Android Pie-based HydrogenOS Beta 2, then follow the link below to download the zip file and flash it via TWRP.