I had guessed 3DO but a Google search turned up an article about one for the NES. I don't know if it qualifies as "usable" though because it can only run a word processor that supports 8 files containing a screen each of text.
Moving up to the Sega Genesis with its 16-bit 68000 that many old UNIX systems were built around: it still only has 64 KiB. But, hey, that’s a lot better than 2 KiB, and it has memory protection. But the 68000 can’t do virtual memory (unassisted) and I don’t believe the Genesis has a timer interrupt, which seems like it’d complicate multitasking (I guess you could run the scheduler when the graphics system raises the vertical retrace interrupt?) but IMO it’s about the minimum level that could run something like what we tend to think of today when we talk about an operating system.