Wash and slice the oranges into thin rounds. Remove any seeds if necessary.

In a medium saucepan, combine the caster sugar and water. Place the saucepan over medium heat and stir until the sugar completely dissolves, bringing it to a low boil.

Add the orange slices to the sugar syrup in two batches. For the first batch, carefully submerge the orange slices into the sugar syrup. Bring the syrup back to a boil, then reduce the heat to a simmer. Allow the orange slices to simmer in the syrup for 10 minutes.

Using tongs or a slotted spoon, gently remove the first batch of candied orange slices from the sugar syrup. Set them on a wire rack to drain away the excess syrup while you work on the next batch. Be sure to reserve the sugar syrup for later use, as it can either be poured over your cake or used for another purpose.

Preheat your oven to 150°C (130°C for fan-assisted ovens, gas mark 2, or 300°F).

Once all the orange slices have been cooked in the sugar syrup and allowed to drain for a minute or two on the wire rack, then transfer them to a baking tray lined with greaseproof paper. Arrange them in a single layer to prevent sticking.

Bake the orange slices in the pre-heated oven for 10 minutes. Then carefully flip the slices over, and bake for an additional 10 minutes to ensure they are dry.

Remove the candied orange slices from the oven and set them aside to cool completely.