AAthens: Is a trip to Delphi worth it from Athens?
It is quite long for a day. You can get enough of the ancient vibe in Athens as well
AAthens: Is Omonia Sq a safe area?
It is safe but not the nicest place to stay. Try looking close to Syntagma Square or Acropolis of Athens. You will find super cheap hostels or Airbnb there. Worth the few extra bucks comparing to Omonoia