Since the OMSI game takes place in the 80's i'd figured out that the Mercedes Benz O303 was a popular during these days. though it was build for Long-distance travels and such I have found some info about the usage of this bus as a city bus.
" One example is seen here in the small Dutch town of Culemborg where it was used as a city bus by local operator Streef Tours. In fact, it was the only city bus and seemed rather big. "