Place cranberries, sugar and vinegar in small saucepan and heat over medium heat until cranberries pop, 5 - 10 minutes.
Let cranberries cool slightly and then transfer to a blender. Puree (being careful to make sure the top doesn't pop off if they are still warm) the mixture until fairly smooth - there will still be pieces of bright red skin. Add the mustard and orange juice and lightly mix.
Transfer the mixture to a medium bowl. Slowly, slowly add the oil while continuously whisking until all oil has been incorporated and the mixture is emulsified.
Use immediately or store in the refrigerator.