The mixture of oil and water can also be called a heterogeneous mixture because you can see both of the parts of the mixture.

Yes, mixtures where all additions are visible are called heterogeneous. Mixtures that look like 1 thing even when multiple things are added (for example juice) are called homogeneous. Fruit juice without pulp would be homogeneous (fruit juice, water, sugar etc...) whereas fruit juice with pulp is heterogeneous because you can see the pulp.