The OnePlus 6 was conspicuous by its absence on the list of supported devices for Google ARCore but it turns out that the ARCore app can be sideloaded on the phone and it works just fine. It is not known when the OnePlus 6 will be eventually added to the Play Developer Console but it should only be a matter of time.
https://www.notebookcheck.net/OnePlus-6-can-unofficially-run-Google-ARCore-apps.309738.0.html