Sorry, I should have said how to get to Crete. It's quite easy since it's the only island (AFAIK) that you can fly to direct from Europe. Lufthansa flys to Heraklion (or Iraklion) from Frankfurt. Otherwise you can go to Athens on any airline and change to Olympic, but I wouldn't recommend it. It's a pain to transfer from the Intl airport to the domestic. It's best to go through FRA.

Changing terminals at ATH is indeed a nuisance, but Olympic's domestic competitors (all, afaik) use the same terminal as the other international carriers, and offer service to the islands. Aegean Air serves Santorini, for instance, although with a bit less frequency than OA.