I finally took the plunge and tried COS today on my 9.
I had to root and install a module to get mobile data working since mine is a 2115 NA variant which was a bit of an adventure but not too bad.
I kept having problems sideloading things because of that built in virus scanner. I couldn't get the cloud login thing to work on the phone, neither phone nor email, but I went on my computer and signed up with Oppo Cloud and that was the end of that problem.
I found myself very frustrated because I couldn't find a suitable recovery for flashing Gapps and then I happened to stumble on a YT video for the 9RT that showed just to go to APK Pure and install Google Play Store. I was skeptical but that indeed was all that had to be done.
There is still some Chinese, especially the universal search when you swipe the middle of the home screen, but it's not really bothersome to be honest.
I was able to get Magisk 24.3 working by pairing it with Universal SafetyNet Fix and Denylist. Apps like Google Pay, Chase, this small screwy credit union I have that won't even let you login if it detects root... All works perfectly. Google Messages is working with Chat features as well (at first it said my device wasn't supported and then I went to check a message and it popped up and activated).
If you want to use apps like PayPal and some banking apps, the device will automatically go into a security protection mode that will complain about the default messaging app and won't let you do anything but close the app or change back to OnePlus' app (and lose Chat). Go to Settings > Passwords & security > System security > Payment protection to shut that off.
Performance is very good. I don't have any complaints here.
I am going to find the module that disables the performance throttling and see if that gets even more but it's definitely way more responsive than OOS.
Oh yeah, if you are like me and don't want 5G draining your battery, you will need a third party app because there is no toggle in settings. I use this one.
This app will help You change network to 5G/4G/3G/2G and stay in chosen network
play.google.com
You don't have to give it any permissions, just click through the setup and then choose Method 2. Change the default string to GSM/LTE/WCDMA (PRL) or anything else you want that doesn't have NR. Annoyingly, for some reason, this resets every time you reboot (it sticks between reboots on OOS) but I don't reboot often so it's not going to be a big deal.
I don't have any plans to go back to OOS. Every app I have works fine and annoying bugs, like auto rotation getting stuck on some of my games and cutting them off, doesn't seem to be a problem here so far. Too early to tell on battery but I wouldn't think it would be a lot worse than OOS12.
ETA: I just looked using Root Explorer and it doesn't seem to have the TPD module that's responsible for the throttling on OOS so yeah, that would be why it's so much faster lol.