So about a year ago I built my first computer and bought a windows 8 OEM disc to install windows on it. I'm about to start building a new computer and I'm wondering if I can just use the same disc.
Same disk yes, but you may need a new product key.
so either way i have to buy a new one?
Not quite. All you really need is a new product key or deactivating your own so that you can reuse it. If you don't want to go through the hassle of pulling your key, deactivating, reactivating, etc., you can go to a reddit forum where they sell legitimate Window keys for a rather cheap price. Be warned, there is the possibility of getting scammed by someone on there. Do this at your own risk!
On the other hand, you can pull your own key and reuse it in another computer. You may have to go the Microsoft forums to get the specifics but it can be done! This way is far safer but very tedious since you have to deactivate you key and then reactivate it, so on and so forth.
I hope this helps you out. If I were you, I would just take the long route, ask around on the Microsoft forums, and do it myself. Better safe than sorry!