OnePlus haven't got much of a track record compared to Samsung (beginning of time) and Pixel (wrote the operating system). They supply a custom distribution of Android, OxygenOS which has reached 5.0.1 (stable) and is based on Android 8.0 Oreo, but this is only on the 3T at present. On the 5T you get 4.7.6 which is Android 7.1.1 Nougat.
I've got the ancient OnePlus 2 which is OK for me. That uses Oxygen 3.6.1 (Android 6.0.1) and I'm not holding my breath for an update soon.
It's hard to tell how good they are at support and updates etc. Users and forums vary between belief that they have found the grail and accusations that OnePlus has ruined their lives by forcing them to buy a poisoned phone. My impression is that they are no better or worse than any supplier. Patches come out every so often and if full updates are slow that's not necessarily a bad thing.
Edit: What he said (points up). I sat there without posting this for an hour or so.
EDITED: 5 Feb 2018 00:06 by WILLIAMA