By Mark Lamport-Stokes OAKMONT, Pennsylvania (Reuters) - Rory McIlroy has already won four major titles but says that a victory at this week's U.S. Open being played at treacherous Oakmont Country Club could rank as his greatest ever accomplishment. All four of McIlroy's previous wins in golf's blue riband events have been achieved on soft layouts where the scoring has been low, very different from Oakmont where the last U.S. Open champion here, Angel Cabrera in 2007, finished at five-over par. "To be able to win on a course like this with the conditions the way they are, it would probably be my biggest accomplishment in the game ... would make me feel like a more complete player." McIlroy announced himself as a potential golfing great with a stunning eight-shot win to claim his first major title in the 2011 U.S. Open at Congressional, becoming the championship's youngest winner since 1923. More...