Step 1 In a medium, heavy-bottom saucepan, combine the sugar with the orange juice and one-fourth cup water. Bring to a boil, stirring occasionally until the sugar is completely dissolved. Continue to cook until the sugar caramelizes and turns an amber color, then remove from the heat and immediately pour into a 13-by-9-inch glass baking dish and allow to cool to room temperature.
Step 2 Heat the oven to 300 degrees. In a large bowl, whisk together the eggs, then whisk in the condensed, evaporated and regular milks. Whisk in the amaretto and vanilla, and then pour the flan base into the baking dish.
Step 3Place the dish inside a large roasting pan (the roasting pan must be big enough so that there is a 1-inch clearance on all sides) and fill it with enough hot water so that it comes halfway up the side of the glass baking dish. Place the pan in the oven and cook 1 1/2 to 2 hours, until a knife inserted near the center of the flan comes out clean. Remove the flan from the water bath and allow to cool, then place in the refrigerator, loosely covered, to chill until cold, preferably overnight. Invert onto a platter and serve immediately.