Preheat the oven to 325ºf / 180ºc.
Grease a 9 inch springform pan with cooking spray or oil.
Make the oat biscuits into crumbs, either place in a bag and break with a rolling pin or use a food processor.
Melt the butter on a low heat in a pot on the stovetop.
Mix together the melted butter and crumbled oat biscuits til well coated. Place the mix into the greased springform pan, pressing flat with your hands to from the crust.
Place in the oven for 10 minutes, then leave to one side.
In a large bowl, mix the cheese and sugar together.
Stir in the yogurt.
Add the beaten eggs in increments, stirring til well combined so you have an even mixture.
Divide the cheesecake mix into two bowls or jugs.
Melt the chocolate on a low heat on the stovetop. Let cool slightly and add to one half of the cheesecake mix and stir to combine. Stir in the black food coloring.
Add the grated orange zest and orange juice to the other half of the cheesecake mix and stir to combine. Stir in the orange food coloring.
Use a large spoon to dollop on the orange and black mixtures in a random pattern. Don't worry if the top isn't smooth, it will even out in the oven and it's quite nice to have a uneven texture.
Bake in the oven for 40-50 mins.
Once cooked, the cheesecake should be set around the edges, but still have a very slight jiggle in the middle. Turn the oven off and open the door to let the cheesecake cool slowly so that it doesn't crack. After 30 minutes, place the cheesecake at room temperature for one hour too cool.
Chill fully in the fridge til ready to serve.