A mainframe operating system is not designed to be used by a general computer user but rather a specialized administrator, a PC operating system on the other hand is designed so that (in theory) anyone can use it.
One of the biggest differences is that a mainframe operating system is usually used to service and cater towards many users at one time. Mainframe operating systems are usually used for Servers. A PC operating system typically is only used and has to create a capacity for one user at a time.
While a PC is generally meant to serve one person at a time using one monitor, keyboard, and mouse, a mainframe is designed to serve multiple users at separate terminals.
The main difference is that mainframe OS has a database server and offers services to its client and other workstations. While PC has a standalone OS and offers no services.
