Things that shouldn't be.

If the oil is truly coming from under the head, the only place there is oil is at the edge of the head on the spark plug side (splash oil only), or the oil passage between the block and head at the rear pass side for the valve train (pressurized). It is not a common problem, but I have seen an issue of the dowel pins being too long after milling head/block, and holding the head up. But this will usually result in water leakage, and lose of compression. Another issue is something (trash/rust) falling out of a head water passage while installing the head preventing a complete seal. I usually fill the water ports in the head with shaving cream before installing the head. Keeps anything from falling out onto the deck surface, and the cream dissolves as soon as water hits it.
EDIT: Possible head bolts are bottoming out do to .100 milling. Whenever I mill I use hardened washers under the bolt head