In a medium saucepan, bring the water, sugar, and fennel seeds to a boil over high heat. Reduce heat to medium, and cook until liquid reduces to about 1/2 cup, about 25 minutes.
Using a sharp paring knife, peel the oranges, removing as much of the bitter white pith as possible. Slice oranges crosswise into rounds 1/4- to-1/2-inch-thick, and place in a large bowl.
Strain the syrup over the orange slices. Stir in lime juice, and zest. Cover with plastic wrap, and refrigerate until chilled, at least 30 minutes and up to overnight. Serve orange slices drizzled with syrup.