You can take route 309 all the way from Osaka, it comes from southern Osaka and leads all the way to Nara (which is a cool place to see, it's one of the ancient capitals of Japan).
Look at the map Image and you will notice that Route309 disappears where they show it. It's a huge tunnel through the mountain. Before the tunnel you take the turn to your right, and just follow it down. Past the sign that looks like 'welcome to some park blah blah' and it slowly gets more twisty. Then all of a sudden there is the badass downhill section.
This is Mizukoshi Touge, basically 'water belt mountain pass'. I met a couple dudes there on a freakin wednesday night and they had stacks of tires laid out lol, made themselves a pit area in the hiking trail parking lot at 2am, nice S13 hatch and SR powered R32