As far as I know, the OEM option has been activated (unlocked) and hidden by the developers on purpose to avoid inadvertently disabling it. So you do not need to activate anything.
This ROM is EOL aka end of life.
Read more here https://en.wikipedia.org/wiki/End-of-life_(product)
Please do not take anything without asking us first. We will 99% allow you to use anything, just ask first though. You cannot use any of our work which hasn't been released in an official ROM release unless given express permission. We are not responsible for anything that happens to your device while using this ROM. By using this ROM you agree with the above terms.