The ST was named using Sam Tramiel's initials, so I always assumed TOS was Tramiel Operating System. It's all Tramiel from Commodore on down.

Officially ST stood for "SixTeen", as it was a 16-bit system.

That was emphasised when the later 68030-based model in the series was named TT ("ThirtyTwo").

Interesting, I'd read somewhere that is was "Sixteen/Thirty-two", as M68k has a 16 bit data bus but is 32 bit internally.

TT was then Thirty-two/Thirty-two as the '030 data bus was 32 bits.

Yours makes more sense though.