OMAHA, Neb. - Oklahoma State will be the favorite when the Big 12 Championships begin at the Qwest Center in Omaha, Neb., on Saturday.    OSU will be challenged at several weights for the title beginning at 125 where undefeated and top-ranked Sam Hazewinkel (32-0) will begin his quest for the NCAA title. Hazewinkel will have to get past eighth-ranked Matt Keller of Nebraska, No. 12 Grant Nakamura of Iowa State and No. 17 Coleman Scott of Oklahoma State.    At 133, Oklahoma State's Nathan Morgan will be the favorite in what may be the weakest bracket of the tournament. Morgan, ranked 10th, is the only wrestler ranked in the 133-pound weight class.    The 141-pound tournament will likely be a preview of the NCAA Championships with three wrestlers ranked in the top five. Iowa State's top-ranked Nate Gallick will receive the top seed, but second-ranked Teyon Ware of Oklahoma and Nebraska's fourth-ranked Matt Murray will challenge Gallick for the title. Add in No. 12 Daniel Frishkorn of Oklahoma State and there are four potential All-Americans from the Big 12.    Oklahoma State's top-ranked Zack Esposito is a heavy favorite at 149. Missouri's Cody Greene is the only other wrestler ranked at No. 19.    Iowa State's fifth-ranked Trent Paulson will be the top seed at 157. B.J. Wright of Nebraska and Missouri's Brad Cieleski are both capable of  upsetting Paulson for the title.    The 165-pound weight class may be the most interesting weight class of the tournament. Third-ranked Johny Hendricks of Oklahoma State is the highest  ranked wrestler, but will be the three seed because he did not wrestle everybody in the Big 12. Tyron Woodley of Missouri will be the No. 1 seed, and sixth-ranked Travis Paulson of Iowa State will be the No. 2. Eighth-ranked Jacob Klein of Nebraska, who defeated Hendricks earlier this year, will be the No. 4.    Chris Pendleton and Ben Askren will renew their rivalry at 174, but first they must get by Oklahoma's E.K. Waldhaus and Iowa State's Nick Passolano.    Nebraska's second-ranked Travis Pascoe and Iowa State's third-ranked Kurt Backes will look to have a rematch after Pascoe won the first meeting. Oklahoma's No. 16 Justin Dyer is capable of pulling an upset and not allowing the rematch to happen.    At 197, Oklahoma State's second-ranked Jake Rosholt will be the favorite, but all five wrestlers at this weight are capable of winning. Rosholt defeated B.J. Padden, the No. 5 seed, 3-2 in their last match. Joel Flaggert of OU, ISU's Trevor Smith and MU's Jeff Foust are ranked 17th, 18th and 19th, respectively.    Steve Mocco will be the heavy favorite at heavyweight. His closest match in the Big 12 was to Oklahoma's back-up Joel Tapler, a 10-3 decision. Mocco  recorded one fall and three major decisions in his other four Big 12 matches. Iowa State's No. 11 Scott Coleman will be the No. 2 seed.    The Big 12 Tournament will provide a lot of excitement not only for this weekend, but also for the NCAA Championships in two weeks.