Unless it's a very old OS, it won't fit on a CD -- you'd need a DVD.  How useful the ISO is depends on the age of the OS -- if it's Windows 10 (the oldest still-supported version of Windows), you're better off using a current install downloaded from Microsoft to install the OS -- it will save you hours of downloading updates.  
I have found a Dell reinstall disk ISO online for my Dell laptop. Am I allowed to burn the ISO into a CD and use it?
Burning a Recovery Disk ISO