In a large Pyrex measuring cup (the 4 cup capacity ones are the best to use for this) combine the sugar, orange juice and orange zest. Whisk until there are no lumps.
Add in the butter then whisk together again until smooth. If it is not pourable, you can microwave it for 30 seconds or so.
Let the mixture sit if you microwaved it, otherwise you can pour it over your cake or loaf.
Notes
You can adjust the consistency of this to your liking by adding more powdered sugar or juice, to thicken or thin it out.