Ö or ö is one of the 4 extra letters used in German. It can be replaced by using the letters Oe or oe. In English language newspapers it is often written as O or o but this is not correct.
Internet addresses are written as "oe" because the internet address system can only understand ordinary English letters.
German is one of the official languages of Switzerland. It has its own dialect (Swiss-German or Schweizerdeutsch). The Ö or ö is used in the Swiss dialect as well.