Depends on what kind of diet.

Seriously, though, there are many things that affect oil absorption when oil is used in cooking, including temperature and general permeability of the food being cooked. (Just as PP said so much more eloquently while I was typing this.) But it doesn't necessarily add as many calories as you might think.
One clear example is popcorn.
Air-popped popcorn, 1 cup popped kernels: 31 calories, 0g fat, 6g carb, 1g fiber, 1g protein (numbers are rounded)
Oil-popped popcorn, 1 cup popped kernels: 55 calories, 3g fat, 6g carb, 1g fiber, 1g protein (numbers are rounded)
The oil adds 24 calories - about .2 tablespoon's worth, or just over half a teaspoon (a teaspoon of oil has about 40 calories).