Hmm; Even if you attempt to backup 'just the OS', the size *may be* an issue. The tool you use will be too.
To recover using such a backup, you need the BOOT info (legacy or UEFI) which is outside the partition table and
the contents of \Windows.
When you absolutely have to use it, just what is the procedure and where are the tools?
I think you should probably get a portable hard drive to do that since most people installed bunch of software on system disk. You should check the size of system disk before you do the backup. If the size can fit with your USB, just go ahead