Apocryphal (I think Bill Porter said this but I don't recall it was a number of years ago): bus is actually the correct spelling, as in
busbar, but engineers in the classic days wanted to separate the spelling from confusion with the word "bus" and all of its various connotations so they took to mispelling it with the extra "s". No idea if that's true or not.
In practice today you definitely see it spelled both ways without much rhyme or reason.
ETA: Wikipedia entry for busbar claims that the term itself is derived from "omnibus" (without citation, though).
Lots of these terms in production with less than clear etymology (I've heard at least half a dozen explanations as to what M.O.S. stands for, none of them ubiquitously accepted).