Trim the tops off the oranges and discard. Also , cut the oranges lengthwise into quarters. Bring 6 cups of water to a boil in a saucepan; add oranges. Bring the water back to the boil then drain. Repeat boiling process twice more with fresh water. Put the cooked oranges, caster sugar and 4 cups water into a sauce pan over medium heat. Cook stirring often, until sugar dissolves and orange rind can be easily pierced with a fork. This should take roughly 30 minutes. Remove the pan from heat and allow to cool completely to room temperature. You should end up with a syrup in the pan. You are now ready to start mixing the cake so heat oven to 170`c. Grease a 10 inch round cake pan with butter and dust with flour and also line the bottom with baking paper. Put aside while you prepare the cake. Whisk together flour, baking powder, and baking soda in a medium bowl and set aside. Remove the orange quarters from syrup, discard any pips and put oranges into a bowl of a food processor. Pulse until oranges form chunky puree. Add the icing sugar, the prepared flour mixture, vanilla, eggs and process until incorporated for approximately 2 minutes. Pour in the olive oil and orange juice and continue to pulse until combined. Pour the batter into prepared pan. Bake for 20 minutes in the oven and allow to cool before serving sprinkled with a little sea salt on top.