In a small skillet, simmer the orange juice over moderate heat until reduced to 2 tablespoons, 6 to 7 minutes. Transfer to a blender. Add the cooked egg yolk, rice vinegar and mustard and process until smooth. With the machine on, add 1/3 cup of the olive oil in a thin stream and process until emulsified. Season with salt and pepper and transfer to a small heatproof bowl. Set the bowl over a small saucepan of barely simmering water and keep the sauce warm; do not boil.