Thoroughly wash the orange to get rid of any waxy residue. Slice off and discard the ends, and slice the rest of the fruit into 1/4' slices.
In a wide pot, heat and stir water and sugar until the sugar dissolves. Avoid stirring after this point to prevent crystallization. Bring to a gentle boil and add the orange slices. After 15 minutes, lower the heat to medium-low. Flip over the slices and continue cooking on low for another 15-20 minutes, until the peel becomes translucent and the syrup has thickened. Place a wire rack on top of a lined baking sheet, and transfer the candied slices on top to cool. Reserve orange syrup for other uses.
Melt chocolate and dip slices halfway. Place slices on a parchment-lined tray and chill.