Keio Takaosan Onsen how to buy tickets

Are tickets available on line and can it be purchased in advance of arrival day?

Advance purchase is not possible. You buy it on site at the time of the visit. Cash, credit cards and IC cards (Suica, etc.) are accepted payment methods.

its an ordinary commuter line. Be careful to select the limited express train (no price difference) instead of a local train. Tested in 2024 by mistake, local train is pain :-), if main intent is to visit the onsen.
Usually during the morning time you can get a direct limited express train straight to Takaosan station, but afternoon and evening you might need to travel little bit on local train and then switch to the limited express back to Tokyo.

BR,
Lauri

I guess you are nur asking about the train ticket but the entrance ticket to the onsen tgat is at Takaosanguchi station.

https://www.takaosan-onsen.jp/index.html

You just buy the ticket at the entrance.

Note that until 13feb some parts of it are closed. Full reopen on 14/feb.