If there are 80 fish in total, then there are 80 fish / 2 = <<80/2=40>>40 blue fish.
The orange fish count is 15 less than blue, so it's 40 fish - 15 fish = 25 fish.
Olaf has then 80 fish - 25 fish - 40 fish = <<80-25-40=15>>15 green fish.
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