Pages 128 and 129 of Highway Buses of the 20th Century by William A. Luke and Linda L. Metler has a photo of a U.S. version of the O-302 with this caption:
"The Mercedes-Benz Model O-302-40, built in Germany, was introduced in the United States in 1968. A number of sightseeing companies bought the new European-style bus. Rawding Lines, the Gray Line Sightseeing franchise in Boston, Massachusetts, was one of these companies. The Model O-302-40, which was very popular in Europe and elsewhere, had a Mercedes-Benz OM355 diesel engine."
The bus in the photo has windows that are taller than the ones on the Wiking model, and it had four round sealed beamed headlights rather than the Euro-style lights on the model, but a least it is close. I have not tried to find any info on the internet for the O-302, but it looks like you might be able to narrow your search to sightseeing lines.
Carter