In a saucepan, boil the orange juice until it's reduced to 1/3 cup, about 12 minutes.
Let cool.
In a microwave-safe bowl, whisk the egg yolks with the sugar.
Whisk in the reduced orange juice and stir in the butter.
Microwave the mixture on high power for 1 minute, until thickened.
Whisk until smooth.
Press a piece of plastic wrap directly onto the curd and refrigerate until chilled.
Line a rimmed baking sheet with parchment paper.
In a microwave-safe bowl, combine the white chocolate with the oil.
Microwave for 1 minute, until the chocolate is just melted; whisk until smooth.
Let cool.
Dry the grapes with a towel and set them in a large bowl.
Pour in the chocolate, folding to coat.
Sift the confectioners' sugar on top and stir until the grapes separate; transfer to the baking sheet and refrigerate for 10 minutes, until the chocolate sets.
Serve the grapes in glasses with the orange curd in ramekins alongside.