Rinse cranberries in a colander, and pick out any that are soft, bruised, or unripe.
Zest, then juice the orange. Put the orange juice in a measuring cup, then add enough water to make 1 cup. My orange yielded 1/2 C of juice, so this is 1/2 OJ and 1/2 water.
Put the water/juice mixture and sugar in a heavy saucepan. Bring to a boil over high heat, stirring to dissolve the sugar.
When all of the sugar is dissolved, add the cranberries. Once it has returned to a boil, reduce the heat to low, and simmer for about 15 minutes, until most of the cranberries have burst and the sauce has reduced a little bit.
Remove from the heat, stir in the orange zest, and transfer to a bowl. Cover, and refrigerate until it thickens. It’s better the next day!
This should be good in the fridge for up to a couple of weeks, and should freeze quite well.