There are two bus terminals in Athens that will take you anywhere you want to go in Greece.
KTEL Kifissou serves the Peloponnese and western Greece. Bus routes 051 (from Omonia Square), 420 (from Piraeus port) or X93 (from Athens International Airport) connect to KTEL Kifissou.
KTEL Liossion serves central and northern Greece. Bus routes 420 (from Piraeus), A10 (from Larissis Station), and X93 (from Athens International Airport) connect to KTEL Liossion.
KTEL: Website / Tel. +30 210 880 8080
KTEL Kifissou: Tel. +30 210 512 4910
KTEL Liossion: Tel. +30 210 831 7186