OnePlus phones no longer use Oxygen OS - it has been supplanted by Color OS, the same as used in Huawei phones.
With the cofounder having left OnePlus (to start Nothing phone!), and the demise of Oxygen OS, and the ending of the offering of "flagship killer" models, the company has entirely rebranded itself these days to be a "full flagship" along the likes of Samsung and Google and Apple, though ofc far behind all of those.
They are half-decent phones, reputedly, i.e. possibly worth replacing the OS on just to get the hardware for the specs, though the other cofounder - the one who remained behind - has a history of alternately playing nice for a few years then betraying the entire user base, then rinse & repeat with newer customers, so for someone who does not want to replace the OS with a custom ROM I would not recommend them.
My own phone's update (7T) was so buggy (it made a minor but noticeable international news sensation) that - I kid you not - I thought that the device might literally explode. I quickly turned it off and it did not, but it got REALLY hot for awhile. After researching, the solution was simple: boot the device into recovery mode and wipe the temporary cache. But WTF OnePlus, you couldn't have had the update do that automatically rather than scare people like that? And I think that melted some internal shielding component b/c even after replacing the OS the phone has never behaved the same - I can no longer hold it in my hands anymore for more than a few minutes at a time b/c it literally hurts and even makes red marks on my skin. My device went from buttery-smooth scrolling - a phone I LOVED, and raved about to anyone who would listen, proudly showing it off - to a piece of crap in less than ten minutes, due to a buggy update rollout. :-(
My update experience seems worse than most, but I am far from the only one, when the company decided that their most popular model - the 7-series - simply was not worth maintaining anymore, as they dropped support for Oxygen OS from the 6-series and older models, and switched to the Color OS for the 8-series and newer models, leaving the 7-series delayed for a long time whether the company would do anything at all for us.
i.e. Carl Pie left for good, solid reasons - not just to start a new thing but to leave the old bad thing behind:-|.