The answer was:

First, I do not suggest the One Day tour on the intercity bus. The total trip to Olympia takes +/- 5.30 hours each way and the time you have left for the museum and the site is hardly enough.

Second, you cannot book the bus tickets in advance. You can buy them, paying cash at the bus terminal of KTEL Kiffissou. The intercity busses have no on line booking engine.

If, however, you decide to try and do the One Day Tour I suggest the departure to Pyrgos and 07.30 am and the return from Pyrgos at +/- 05.00 pm. The connection to Olympia and especially from Olympia can be done either on the local bus or a taxi from Olympia to Pyrgos if you want a little extra time at Olympia. The cost of the taxi one way is +/- 25.00

When you arrive in Athens you go to the KTEL Kifissou offices and buy your ticket to Pyrgos in advance or be at the terminal 30 minutes before the bus departure and buy your ticket then.

Another problem that you have to take care of, is your return. As soon as you get to the PYRGOS bus station you visit the ticket counter and buy your ticket from Pyrgos to Olympia and at the same time book your return from Pyrgos to Athens.

You risk of course to find the return to Athens, fully booked. This is a problem.

If you want to call the bus terminal in Athens the phone No is 210 5134110 and in Pyrgos is 26210 20600