For Live Caller ID to work, try changing battery optimization settings and set Truecaller as the default Caller ID App on your OnePlus device:
- In your phone's Settings, go to Battery > Battery Optimization
- Find Truecaller and choose "Don't Optimize
2) Battery optimization
4.) Turn off
Steps to set Truecaller as default Caller ID and Spam App:
Go to native device settings > Default caller ID & spam app > Truecaller.
Note, this is required for blocking calls too.