It really depends on the weight of the orange. Most items now have calorific information, which is very easy to work out. You just need to weigh things and work out calorific values.
I think all of that would be well under 300 calories.
100mls of Semi Skimmed Milk contains 49calories. A cup could be any amount, but if you estimate that it's 125mls (49 cals /100mls x 125mls = 61.25 cals)
1 teaspoon of white sugar has 17 calories, A tablespoon is probably about 2-2.5 teaspoons. (34 - 42.5 cals)
100g Orange flesh only is 37 calories. Again, you need to know the weight to work out accurate calories, but let's say it's 100 grams.
100g Porridge oats is 368 calories, but porridge is not heavy in weight, so you would probably only be using about 30g (368cals / 100g x 30 = 110.4 cals)
Aproximate calories for the things you mentioned is 243 calories.
I would reccommend buying a book called the "Collins Gem Calorie Counter". It contains the nutritional values of most foods, including fast foods and branded items. It costs about �3.99 from most bookstores.