Mt. Odaigahara is located in the Yoshino-Kumano National Park. It is also one of 100 most famous mountains in Japan, and designated as UNESCO Eco park.
This mountain consists of vast plateau and rain forests. Many kinds of mosses and plants are growing up because it is  the most rainy region in Honsyu island.

The altitude is approximately 1, 600m. Nevertheless, you can go to the parking lot near the mountaintop. It takes about 4 hours from the center of Osaka by car.

Daijagura is the steep mountain  standing out along a cliff from the deep valley, so nobody can approach it. On the other hand, characteristic landscape of the blighted trees by natural disasters are spreading on the top of plateau named Masaki-Touge for decades.
A day trip is possible, but I recommend staying overnight and watching a night sky and the daybreak.