Is there only a difference of formality when using Oyasuminasai or Oyasumi=? As far as I know they both stand for 'Good Night'. I am not an expert in Japanese but my educated guess would be that Oyasuminasai is the more formal version of Oyasumi? -->because of the suffix 'nasai'.
What is/are the Kanjis for Oyasumi? @Blaveloper would you be so kind to introduce some sample sentences. That would be appreciated.