What is the supplied o-ring for and where does it go? Sorry for the newb ?

check your other post

The o-ring goes over the extractor spring. With the new spring and o-ring in place, press down on the assembly as you put it back in. It's going to take a little more effort to get back in with the new components.

should shoot fine without it

Try your BCG without the optional O-ring first and decide if you have enough extraction force.

The extractor spring would work fine without the O-ring, when your extractor spring gets worn and have less power then it's time to install the O-ring to increase the extractor force.

I only use my BCM extractor upgrade kit on my carbine gas system AR, on my other midlength and rifle gas systems I only use just the O-ring with a standard strength extractor spring, don't need the extra power extractor springs, it's enough for me to be reliable.

HTH