Olga has half as many blue fish as yellow ones, which means 12 * 0.5 = <<12*0.5=6>>6 blue fish.
Green fish are twice as many as yellow ones, so in total there are 12 * 2 = <<12*2=24>>24 green fish.
So all the fish sum up to 12 + 6 + 24 = <<12+6+24=42>>42.
#### 42