Yeah, they most likely forced the post in there and it held due to the out-of-round clamp. BCrider's method of expanding the oval back into a circle is right on. You'll need to find a smaller-diameter post for the procedure, or a pipe of some sort.
Once you've expanded the top back to round, make sure the post fits snugly all the way down without rocking. It could be that the post the bike came with is too small and tightening the clamp-bolt caused it to distort.