No, you won't have a lot of problem.
OKW is our home resort and we usually stay there for about 10-14 days at a time, twice a year. It's very seldom that we have had any problem getting on a bus with DD's wheelchair. Since parking is very convenient, a lot of people do bring cars to OKW, which may help the buses not to be that crowded.
There are lots of bus stops, but that means you are never that far from a bus stop. Pretty much all the WDW buses are wheelchair accessible. The easiest to get on are the buses with ramps, but you will still see some of the older buses which have lifts.
You can find more information about bus transportation in one of the posts of the disABILITIES FAQs thread. You can follow the link in my signature and then look in post #1 for an index to see which post to find the information you are looking for in.
Another really nice thing about OKW is that there is boat transportation that you can use to go to and from Downtown Disney. The boats are accessible - you will see stairs that go down to the boat dock, but follow the walkway past Goods to Go and around to the right of the pool. THere is a ramp there that goes down to the dock. A CM will come and get you to board the boat. In our experience, we recommend having the wheelchair/
ECV user and one companion go to the ramp and the rest of your party go down the stairs so you can tell the boat CM that they need to bring the wheelchair/ECV down to the boat.