If you are visiting from Osaka for one or two days and buy the Koyasan – World Heritage Ticket (which I strongly recommend you do) bus travel within Koyasan for two days is included as part of that ticket.
If you arrive into or depart from Koyasan Station you are not permitted to walk along the road from the station towards the town centre. You must take the bus to at least the first stop at Nyonindo. The only way around this is a long trek via Damion which will take over an hour.
Should you need to buy it separately, a one day bus pass costs Y830 (Y420 for children).
The buses are of good quality and have a reasonable frequency. You can download a timetable from here:
http://www.nankaikoya.jp/en/pdf/ (select ‘Map of Koyasan’).
There is much debate as to whether or not the bus pass it worth it.
Personally, on my day-trip I had a Koyasan – World Heritage Ticket so no decision was necessary. As it was, I took three buses – the first one from Koyasan Station to Ichinohashi-guchi for Okunoin-mae (around 5kms), one from Okunion-mae to Karukayado (back into town – 1.4km) and one from Daimon back to Koyasan Station (4.5 kms). Had the weather been better I would have walked the second sector rather than taking the bus. Had I bought three tickets (correct change required) it would have cost me in excess of the cost of a day pass. The two longer trips by themselves would have cost Y760 for both (July 2016 price).
The walk from Okunoin to Garan is about 2kms with an additional 1 km to Daimon. Remember that on top of any walking you do around town you will clock up 3-4kms within the Okunoin.
At the end of the day unless you are on a very tight budget and only plan on taking the bus between the town centre and the station then it is better and a lot more convenient just to buy the bus pass.
The bus pass can only be bought at Koyasan Station.
Apologies for the quality of my photographs accompanying this review. The weather when I visited was not conducive to my wishing to spend much time photographing buses. My final two photos were taken from the bus between Koyasan Station and Nyonindo and, no, they have not been converted to black and white!