This is Argiope bruennichi, the Waspspider or Zebraspider. I am also surprised it has reached Oeland. 10 years ago they were first observed in the south of Holland, now they have expanded their territory that far north already. They prey mostly on grasshoppers, so they build their web at grasshopper-rich places some 40 cm above the ground in between the grass.
Do you have these spiders in Sweden? I thought, they exist only in the warmer parts of Europe (but they are spreading quickly, also here in Austria they can be found in places where it would not have been possible a few years ago).
