Please bear in mind tapi Kopi"o" kosong actually is not kosong. You can check back the making of Kopi-O.
Sifu actually dumb in Butter/Marjerin and sugar (different level according to resipi). The bean will roast until dark and dried.
This is Nanyang Style Coffee.

Western coffee process quite simple, only Roast the bean until dry without add any ingredient.

Sugar got a little, that's why you can see black color caramel (which is meaning of -O)_. But don't worry about, we all need some Carbohydrate. This Kopi-O just enough for your metabolism activity.
Dont add in Sweetened Milk, or Sugar.

I try google and result is this.
How is Kopi O made?
Kopi O': Coffee beans are roasted at a very high temperature, often until the beans are dark and glistening, which gives it its bitter, sour- charcoal flavour. Margarine, sugar and sesame is often added while roasting, giving it its rich aroma while diminishing the intensity of taste.