Oil is a very common medium for painters. It dries slowly which allows you to blend and layer colours easily. Acrylic is a different ball game. Acrylics generally dry quickly which can keep your colours pure (beause layers dry before they can blend) I often do an acrylic underpainting and oil on top. It is important to note that the reverse is not recommended. The rule is 'fat over lean'. (Oil over acrylic)
My Dad usually starts with a dark acrylic underpainting and then adds brighter oils on top. He will leave areas of the underpainting exposed- Check out our youtube video on underpainting.