Once you are on the bus at OKW depends on if you are at the first or last bus stop. It takes about 8 minutes for the bus to run the whole OKW route, so if you assume you are the last bus stop, it would take approximately 12 minutes from that time until you arrive at MK. The OKW bus stop is very near the entrance, so then you would just have to figure your walking time to the restaurant. If you have an 8:10 ressie, I would leave OKW by 7:30. Be advised that it is easiest to figure out the bus schedule if you assume the buses leave for each park every 20 minutes starting on the hour. If you are near the last bus stop (Hospitality House) you would want to be at the bus stop by 7:20-7:40.
If you are going to Epcot, the timeing will be similar to MK. MGM is about the same too, but it is a bit farther to walk in to the entrance. AK is the one that takes the longest. The bus often stops at Blizzard BEach on the way to and from AK, so allow 45minutes there.
