Preheat oven to 170°C (340°F), conventional setting (no fan).
Cut the whole orange into chunks and blend until smooth.
In a large bowl, whisk eggs and sugar until combined. Add oil and blended orange; mix well.
Sift in flour and baking powder, and fold until smooth.
Pour batter into a greased 30 × 10 cm loaf pan.
Bake for 50–60 minutes. Start checking with a toothpick at 50 minutes.
Bring orange juice and sugar to a boil in a small saucepan to make the syrup.
Once the cake has cooled slightly, pour the hot syrup over it and let it fully absorb.
Once cooled and set, decorate with melted white chocolate or glaze.
Slice and serve.
Notes
Worried about bitterness from the whole orange? Don't be — it isn't bitter at all. If you'd still like extra peace of mind, remove the white pith before blending.