What component of a computer system holds the operating system when the computer is not running

The same thing that holds the operating system when the computer is running.

It is usually a Hard Drive (HDD) or a Solid State Drive (SDD). I would recommend storing the operating system on the SSD because it loads faster, but a lot of computer's don't have SSD's which is why they are stored on Hard Drive's.

The mass storage devices. Usually a hard drive (HDD) or a solid drive (SSD).