"OEM" windows question

I see that it is cheap. I see people complaining that they can't do fresh installs. Which sounds really shitty.

I want windows 7, I can do windows 10 I guess, but what should I go with so that I don't get fucked?

Thanks, r/buildapc. With your help and my dedicated effort, I may one day learn all the things that I never knew I needed to know.

J/k, I know better ;(