Unfortunately, the automatic install feature is not compatible with the OnePlus 6 and OnePlus 6T.
What does this mean for me?
This means that if you have a rooted device, you'll have to install the update yourself in TWRP recovery (and don't forget Magisk afterwards).
Users of non-rooted devices are unaffected, if you don't have root just follow the install instructions present in the app (and use Settings -> System Updates -> Local upgrade to install the update).
Why is this discovered so late?
Since none of our admins own a OnePlus 6 or 6T, we never saw ourselves that this feature was broken on these devices. Other users reported that the device just showed the recovery mode, and from the logs we saw the installation was never even started on them. This made us realize that the recovery mode could not perform the installation on them.
Why does it not work?
Normally, the app generates an install script, which then gets executed by TWRP recovery to install the update. However, the OnePlus 6 and 6T have a so-called A/B partition scheme. That means that there are actually two copies of the operating system present on the phone, and while updating you switch from the one to the other.
This scheme allows super fast updates and also offers error recovery if the installation of an update fails (by starting up from the second partition).
However, these devices do not have a cache partition. That's why TWRP recovery cannot execute the install script on these devices: It assumes the script is placed on the cache partition, but it's never going to find it there.
Also, the recovery mode does not (yet) support loading the installation script from an alternative location. However, with TWRP recovery being open-source software, you (or some other developer you know) can add this functionality to TWRP. A bug about this feature missing is available at https://github.com/TeamWin/Team-Win-Recovery-Project/issues/1359, so refer to this (or send this link) when implementing this feature.
I apologize for the inconvenience and hope you still enjoy using Oxygen Updater.