MTH made a number of boxcars for the Kennywood park, I believe (not sure how many, but more than one or two I think).
For rides themselves, Lionel also made a bunch (Carousel, Ferris Wheel, Pirate Ship, Scrambler, Swings, maybe another I am forgetting?).
There is also a company called Coaster Dynamix that has made a bunch of roller coasters and other amusement park rides. I've not actually purchased any of these, but I've seen their displays at York many times, and they seem pretty impressive.
Sorry if the additional ride suggestions weren't really wanted. I re-read and now realize you really only asked about rolling stock. I'm leaving it in case anyone else reading is interested.
Pride Lines made some trolleys that were sort of open air, not sure if any in O Gauge are exactly what you'd be looking for. A lot have Disney characters in them. Some have just open windows so you can see inside, but there may be a few models with bench seats that would be entered along the sides of the car (a simple web search showed me a few in this style, but they were Standard Gauge).
-Dave
