What will cause oil in the coolant on a n14 cummins diesel engine?

Bad head gasket?

Warped head?

also try looking at the oil cooler , when they get a leak in the core it dumps oil into the coolant , is so have fun when you try to flush it all out , we use dishwashing soap in the radiator then flush well with water a time or two before you put anti freeze back in. Good answer!