We were at OKW around the same date range ( Dec 18 - 26)this past Decmeber, the parks were moderatly crowded at the beginning of the trip steadily increasing as Christmas got closer and maxing out on the 23rd.
As far as weather goes be prepared for anything, some days were 80 while others were around 60 and a couple of nights were rather cool. Best advise is to always keep a sweat shirt or jacket handy if staying in the parks after sunset.