Bring a saucepan of water and the orange zest to the boil over a high heat. Reduce the heat until the water is simmering, then simmer for 15 minutes. Drain the zest, replace the water and repeat three more times (this will remove the bitter taste from the orange zest).
Drain the orange zest and transfer to a food processor. Blend the zest, orange juice and half of the oil to a smooth purée. Continue to add the oil, pulsing after each addition, until the purée reaches the consistency you desire.
Season, to taste, with sugar, salt and pepper, then cover and chill in the fridge until needed.