From Kawaguchiko you can take a train to Fuji Station (5 minutes, 210 yen, 2 departures/hour) and then take a bus from Fuji Station to Oshino Hakkai and Gotemba.
Roughly every other bus between Kawaguchiko Station and Gotemba (approximately one per hour) makes a small detour via Oshino Hakkai (see timetable https://www.japan-guide.com/bus/gotemba.html). Get off at the "Oshino Hakkai" bus stop (some buses bound for Uchino and Hirano use the "Oshino Hakkai Iriguchi" bus stop, instead; not to be confused with the more distant "Oshino Iriguchi" bus stop).
You can check train schedule here: http://www.hyperdia.com/ (There is also a HyperDia app for smartphones).
Hope this can help and have a nice trip.