OnePlus ship their phones with OxygenOS as an operating system. It's their version of stock Android.
Unlike many other mobile phone vendors, who add or replace many parts of the Android system, OxygenOS tries to stay as close as possible to the stock version.
Sure, there are still couple of key apps replaced, but some of them are put of necessity as Google restrict those parts only for their own phones. The main differences beside those apps are mostly minor improvements. OxygenOS for example offers some additional configuration options that are not available in the stock version.
While the Android 10 worked fairly well on my old Pixel phone I think it wasn't fully compatible anymore. There were many issues with it. But there were also apparent issues due to the new version of the OS and it's interoperability with 3rd party apps. The new gestures for example weren't compatible with custom launchers. So it was a nice surprise when those same gestures worked on OxygenOS with my preferred custom launcher.
Overall it feels like a better version of Android. Mostly thanks to those cudtomization options. The base android is a good system, but especially lately it has been dropping the flexibility with those configuration options. It's nice that there are alternatives that don't go overboard rewriting everything, but just adding what is actually missing from the system instead.