I would say easiest to get to from osan would be all of the climbing in Seoul and probably Ganhyueon and Yeongso Pokpo. I remember Yeongso being one of my favored travel destinations due to only having to take one train to get ther and it only being about 2.5 hours. Outdoor walls are also doable during the work week. I used to go to Tuoksome resort outdoor climbing wall after work in Seoul. There is also a gym in Osan city.
Long awaited home coming.