Like the others have said, whether you have/need an OB depends largely on the model of care you feel most comfortable with.

For my first, I was initially scared and saw childbrith as a possible life-and-death moment for me and the baby, so I insisted that I needed an OB. He was the "expert", after all (DH thought I was being ridiculously paranoid but let me go on my merry way). But during the pregnancy I came to feel that birth was natural, unlikely to lead to death, and a could be managed in a non-medical manner. But I had done my cash already and ended up in my chosen private hospital and OB-led birth. It was "safe", medicalised and produced a good result.

By the time number two came around, I was a full convert to the "birth is natural" vibe and went though a midwife-led model of care with the Family Brith Centre at the Mercy. It was a totally different experience, but the care provided was no less "sound" by virtue of it being delivered in a public setting.

This time I am planning a homebirth with the Sunshine Hospital's HB program. Again, a totally different model of care, but one which is no more risky to me or the baby than having an OB in a private hospital (in fact, on many measures, it is a SAFER one!)

So I think it comes down to comfort levels. If you are fearful of the birth process, you probably will want an OB to steer your boat - if you believe in your heart it is a natural thing that your body knows how to do, you will most likely choose a midwife-led model of care. But your level of comfort has A LOT to do with the level of education you seek out about pregnancy and birth.