The greens were redone around 2001-2002, and the course was closed for a year. The results were what they've been talking about all week - reasonable greens, but more difficult green surrounds. The bunkers became deeper and narrower and there were more chipping areas.
I think Oak Tree has some really good holes and some not-as-good holes. Specifically, I think 1-7 are all quite good; 8-11 are not up to the same standard; 12-16 are excellent; and 17 is OK. I think #18 this week - which normally serves as #9 - is unappealing. A good test, yes, but just not a hole I ever liked playing.
Understandably, the best holes fall over the best land.
How hard is it? Uh, it's really hard. Not impossible but fairly close. Harder than the Stadium Course at PGA West for sure. Although, from y experiences at OU, I can assure you that it's much harder in the winter when the grass is dormant and it's 45 degrees with the wind blowing from the other direction!