Dallas, TX.   Oklahoma State senior Charles Walker was selected Big 12 Wrestler of the Week as announced by the conference office Wednesday.        Walker has led the Cowboys to a 3-0 start this season and the nation's top ranking after OSU posted wins over Missouri, defending national champion and previously top-ranked Iowa and No. 6 Oklahoma.        The Geary, Okla., native is 5-0 this year in the 133-pound division, which includes a 3-2 upset win over defending national champion Eric Juergens of Iowa. Walker's win over Juergens moved him from sixth to third in the Amateur Wrestling News individual poll.        Walker also racked up a  4-1 victory over Oklahoma's 10th-ranked Witt Durden on Dec. 10. Last season, Walker defeated Durden in the 133-pound consolation finals at the Big 12 Championships in Lincoln, Neb. He finished the year ranked seventh with a 30-6 overall record and 14-4 mark in duals.