This recipe contains a raw egg yolk. Though the risk of salmonella infection from consuming raw eggs is low, using a pasteurized egg eliminates the risk entirely.
Combine the olive oil and the vegetable oil in a liquid measuring cup. In a small bowl, whisk the yolk, mustard, and 1 Tbs. of the oil mixture until thoroughly combined. Whisk in the remaining oil mixture a few drops at a time. As the mixture begins to emulsify, add the oil in a slow stream, whisking constantly, until all of the oil has been added. Don't add the oil too quickly and be sure that the emulsion is homogeneous before adding more oil.
Stir in the orange juice, chives, lemon juice, garlic, zest, 3/4 tsp. salt, and a few grinds of pepper. Season to taste with more salt and pepper. If the aïoli is too thick, thin it with additional lemon juice or warm water.