Strictly speaking, this definition does not include any other functionality, so it does not apply to more complex OSes, such as Microsoft Windows, and is more appropriately used only for older generations of operating systems.
Without permanent storage, program and data entry is done at front panel switches directly into memory or through a computer terminal or keyboard, sometimes controlled by a BASIC interpreter in ROM.
By the time IBM announced the System/360 mainframes, the concept of a disk operating system was well established.
Although IBM did offer Basic Programming Support (BPS/360) and TOS/360 for small systems, they were out of the mainstream and most customers used either DOS/360 or OS/360.
AmigaDOS also mostly resides in ROM, as a part of a Kickstart firmware (a few select versions are also loaded from disk).