In a medium saucepan, combine water, sugar, orange zest, and orange juice.
Bring the mixture to a boil over medium heat, stirring to dissolve the sugar.
Add the fresh cranberries and return to a boil. Reduce heat and simmer for 10-15 minutes, or until the cranberries have burst and the sauce has thickened.
Remove from heat and let cool. The sauce will thicken further as it cools.
Notes
Can be made ahead and stored in the refrigerator for up to a week.