By measuring the area produced, one can get a simple estimate for the size of each oil molecule and thus Avogadro's number. Thus the vast majority of oil molecules stay out of the water, though as many as will fit will hang on to the surface water molecules that do not have a full complement of partners. One can make an analogy to a high school dance, where generally one will find that the most popular boys and girls will dance with each other almost exclusively. It is not that the less popular members of the class do not want to dance with the popular students, it is just that the popular students have their choice of who to dance with and, since they also want to dance with popular students, they pair off. The less popular students, rather than have no partners, will naturally pair off as well.
Water has many unusual properties.
Another consequence of this decrease in volume upon melting is that the melting temperature of ice decreases when one increases the pressure on the ice. An increase in pressure on a sample of ice can be partially removed by melting the ice and thus lowering the sample volume. It is often claimed that one can skate on ice because the pressure of the skate causes the ice to melt, thus dramatically reducing the friction between skate and ice. While this makes a good story, it is not quite correct. While one can imagine that the force is concentrated in a somewhat smaller area, the effect of pressure alone is clearly enough to shift the melting temperature of the ice by at most a few tenths of a degree.