Oberon one could guess by seeing it's an OS and its implementation language, tightly coupled under the same name, that Oberon's ABI uses Oberon.

Loosely defined, most embedded Forth systems act as an OS and they tend to use the system's assembly or the Forth system itself as the ABI.

PC-DOS derivatives obviously use x86 registers and interrupts as an ABI - that's more assembly and machine language oriented than C.