OS X help

I don't know anything about Windows, but I would guess that it would depend on how Windows was installed. In any case, it also depends on whether or not the recovery partition was deleted (if they deleted Mac OS).

So, to start, try to boot into internet recovery. If that works, you can delete the Windows and reinstall Mac OS. Make sure you format the drive Mac OS Extended (Journaled) and GUID partition scheme.

https://support.apple.com/en-us/HT204904

https://support.apple.com/en-us/HT201314

I bought it second hand and they never installed IOS on it, only windows 10.

In addition to babowa's recommendations, please be sure to reconfigure that Mac as your own, by following these instructions: What to do before you sell, give away, or ... - Apple Support.

The seller should have performed those steps before selling it. Unless they are followed, you will encounter various problems. You will need to start with Step 6, but ideally the seller should have performed the others prior to selling it to you.

I don't know a thing about Windows either, but it seems to me that you are required to own a license to use it.