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)
A tell tale McKeown painting usually starts with a dark acrylic underpainting with brighter oils on top. This leaves areas of the underpainting peeking out from under the details. Check out our tutorial on underpainting in the link below.