I had updated to OS12 on my OP9 Pro and I hated it. There are now too many useless features, not to mention a few of my most used features disappeared (such as the seconds on my clock in the notification bar, and the quick features by long pressing HOME, RECENTS, & BACK buttons.) So I used the MSM Download Toolkit to flash back to OS11, and I am going to use 11 until T-Mo releases 13.
Here is the complete list of OP devices and their respective MSM Download Toolkit(s) associated with them. *Remember to always use the toolkit for YOUR device model - for they are not interchangeable. You will get a failed result EVERY TIME.* This also works for BRICKED OnePlus devices.
https://onepluscommunityserver.com/list/Unbrick_Tools/
For the specific T-Mo OP9 Pro device, I have the ZIP file available to download along with step-by-step instructions on how to Flash Back to Android 11 on OP9 Pro - inside of my Google Drive folder :
https://drive.google.com/drive/folders/1PbVwsUqW8Zh1sJLgl4Uf-YMkzvr8PJt8?usp=sharing
