The measurements of the bowls are usually in reference to the outside dimensions. The rim itself is typically about an inch wide (give or take), so the inside diameter will be about 2" less than the stated dimensions - a sink that's listed as 17" x 14" will have an interior dimension of (roughly) 15" x 12".
You should ask the dealer that sold you that top what particular sink (brand and model) that was cut out for. The basic Kohler oval undermount sink is the Caxton, and it comes in a few different sizes, but none that would fit particularly well for the dimensions you gave; The cut-out in your top is a little too long for the Caxton to fit properly. You'd have a 1/2" recess at both the front and back, and no recess at all at the sides. You want to have a little bit of a recess all around to ensure none of the unglazed rim is visible from the top.