Turn on the player and eject the disc tray.
On the remote, type 2812, followed by enter.
This gets you to the engineers menu.
Press DOWN until you reach COUNTRY CODE and press ENTER.
The current region code will change to four dashes (----)
Input one number (0=Region Free, 1=USA 2=Europe, etc) and press ENTER again.
The new region code will now be displayed.
You can exit by just pressing the eject button once more.

OP PROH ON is referring to operator prohibitions.
Allegedly, disabling this will let you skip through copyright warnings and so on.
I have yet to pull this off but then I am more interested in getting it to play S/VCDs reliably, without having to hold down the MENU button on the machine, while pressing one of the remote keys (I prefer '5' as that is the one I first succeeded on).