bus service in Peloponnese

Try Rome2Rio as a starting point to get some ideas.

Don't interpret the RS guidebook destinations in Greece as a single trip that can be done in a few days where you could easily go from any place to any other in a few hours.

To my knowledge it is not possible to go directly by bus from Nafplio to Olympia.

From Nafplio to Hydra it's a first bus to Kranidi, then another from Kranidi to Porto Heli, then a ferry to Hydra.

The regional bus service in Nafplio is KTEL Argolidas. Although I suppose it won't be of any use, here is the website:

https://www.ktelargolida.gr/en/homepage-en/