Used a seasonal bus from Yamato-Yagi station (大和八木駅), only one per day departed at 8.15 AM. A long trip up to Oodaigahara, arrived at 11.16. Explored also Nishi-Oodaigahara, it's not worth because of lack of time (the only bus back to the station left at 4.00 PM) and because you have to pay the entrance fee and book in advance. I wasted time and money, nothing really special. One of the less interesting Hyakumeizan (100 famous mountains of Japan).