Fillychi :
Simple question:
If my hard drive fails or dies, and I buy a new one,
do I need to buy a new OS as well if my old OS was OEM?
Generally, no.
Where did the OS come from? Preinstalled, or something you bought?
If preinstalled, create the DVD set that the Factory Reset partition will let you make.
Document the license key
If something you bought, document the license key Print it out and maybe tape it inside the case.
Don't lose the DVD. Maybe make a couple of copies..