It is best practice to install your Windows System drive (C:) onto a SSD. This way your system appears as being really fast. You also install all apps (and games) to C: as long as there is space available. This way your apps start really fast and load their app data really fast.
You will store your personal data not on the SSD C: but on additional disks you add to the system as D:, E:, etc. Usually you will make one drive letter for each hard disk. You have 1 SSD and 2 HD's, so with this schema you will end up with C: (SSD), D: (HD 1), E: (HD 2).
For backup, you make image backups of C: and file level backups of D: and E:. This way, you can restore your system drive from image if C: fails and your system is immediately able to boot again. And if D: or E: fails, you are still able to boot from C:, and you are able to restore the destroyed files using your running system.
For image backup, use the Windows built-in image backup, which can be scheduled, for example weekly. This runs in background, so I never forget about it. It just works. Restoring this is simply booting from the Windows installation media (DVD or USB Stick) and choose system restore.
For file level backup, use File History, which is available since Windows 8.
If you use this concept, it's clear that you will save your OBS videos to D: or E:, depending on how you distribute your application data to these two hard disks. You want to choose your fastest drive for that, because this way your postprocessing work (recoding, editing) is done fastest as well. Don't save these to your system drive, because that way you bloat your image backup with often-changed temporary files.