I don't know about Oslo, but in Munich, where winters are full of snow, plenty of bikes are still around. I certainly continued commuting to work that way, and it definitely wasn't "young people sporting bicycles in summer" for me, it was just my regular means of transportation, totally unsporty.

Many more bikes out in summer, yes, but, well, many more people outside in summer in general.

As for groceries: Supermarkets are pretty well distributed in Munich's residential areas (which I don't think includes the city center, by the way), often so close that taking your car might take you longer.