Displaying a still image for a prolonged period of time may cause an image sticking. This phenomenon is normal character for OLED panel. 
Avoid displaying a fixed image on the TV screen for an extended length of time. 
Or you can clear such image retention on-screen by activating this function.

 (Home) > Settings > All settings > Picture > Expert mode settings > Clear Residual Image

The TV will be switched off until the process is completed. 
If you are recording something, the process will be paused, and restart until the recording is completed. It might take more than 1 hour. 
If you turn the TV on during the process, it will be stopped and will be restarted automatically when the TV switches to standby at a later time. 
While processing, a white line may appear on the screen. This is normal when clearing on-screen image retention.