This is probably ワンマン
(note ン
instead of ソ
). ワンマン
is wasei-eigo from "one man". For example, ワンマンショー
(one man show), or ワンマンライブ
(meaning a live performance with just one band, not necessarily one person).
In the context of buses, ワンマン
means that a bus is being operated by a driver alone, with no conductor (i.e. every bus nowadays). It used to be a rule to have this sign if there was no conductor.
So this is not a phrase ("one man doorway"), but a two-purpose sign showing the doorway and stating that the bus is ワンマン
.
Of course, this being FLCL it's possible that it actually does say ワソマソ
, as a joke. A Google search reveals that it is sometimes used jokingly.