Mike,
I would contend that #s 9 and 18 at Oak Hills are fundamentally different holes. #9 is a level 155 yards from the back tees (135 middle tees) to a small green, at least half of which is covered by deep bunkers in front, requiring a stop-it-quick short iron. #18 is a downhill 198 yards (169 middle tees) to a larger green that slopes from front to back and is more open in front, with difficulty more on the sides than in front, requiring a controlled mid-iron with more hang time. Both are good finishes to each nine.
As for how much of Tillinghast's original work remains, I'm not sure but I know the course has been revised at least three times (in the 60s, 70s, and 80s for sure), and three holes (I believe #s 15, 16, and 17) were added by Joe Finger in the 60s. Still, the course has an "old-style" flavor and the holes around the clubhouse (1,2,8,9,10,18) are all solid (I especially like the stretch of 7-10: 460 uphill, 310 tight, 155, 506 reachable if one carries the right trees on the 2nd shot).