How should I use ohayō, konbanwa and oyasumi? As a greeting or as a goodbye?
- おはよう（ございます） ohayō (gozaimasu) is used like "Good morning" as a greeting
- こんばんは konbanwa is used like "Good evening", also as a greeting
- おやすみ（なさい） oyasumi(nasai) is used like "Good night", said when parting with someone and either party is heading for bed (or back home)
It is completely parallel to the standard usage of the phrases in English. (It is not possible to use konbanwa like the "old-fashioned" "Good day (to you, sir!)" or "Good evening" as a form of saying goodbye.)