In a double boiler, over medium heat, whisk together the egg yolks, sugar, orange juice, heavy cream and orange zest.
Cook, stirring occasionally, for 15-20 minutes until it thickens.
Strain into a bowl and immediately add the butter. Stir into the curd until the butter is completely melted and combined.
Immediately pour into the Oreo tart shells. If you happen to have extra, cover it with plastic wrap and refrigerate for up to 3 days.