One thing you might want to consider is using a clipping mask. This isn't the best example, I just grabbed two images from my current trip.
Put your background on the bottom. Put the face you want to add on top. Put a blank layer in the middle. On that blank layer, draw your oval; set a feather before you draw, then fill the drawn selection with a color. Now, group the face-layer with the middle layer. Shazam, the only thing that shows is that part of the top layer that is 'lined up with' the clipping mask.
Now you can adjust the mask (transform or move tool) to change what you want to show. If you don't like your feather, re-do it.
Just an idea.