What was the name of the first operating system on IBM PC?

DOS: Disk Operating System

TOS: Tape Operating System, most likely came first.

It used 1/2 inch 7 channel tapes on computers without harddisks.

Before that IBM had a system called IBSYS on their 7090 & 7094 computers, but it was more of a Batch Monitor than a real Operating System (it only automatically ran jobs one after another in batches).