They are made by using two spoons to scoop a certain amount of dough and dropping the dough into a deep fryer filled with hot oil. This way, a sphere-shaped "oliebol" emerges.
The dough is made from flour, eggs, yeast, some salt, milk and optionally some sultanas or currants and sometimes apple pieces. The dough needs time to rise for at least an hour. Oliebollen are usually served with powdered sugar.
(From:http://en.wikipedia.org/wiki/Oliebollen )