Tailback Spencer Tillman scored a late second-quarter touchdown and Tim Lashar kicked two field goals as third-ranked, Orange Bowl-bound Oklahoma defeated No. 17 Oklahoma State, 13-0, in a Big Eight Conference game in an icy Stillwater last night.

The game was played in miserable conditions, with temperatures in the low 20s and the wind chill near zero. Intermittent rain and sleet left the field covered with a thin glaze of ice.

Tillman's three-yard burst 1:09 before halftime ended a 33-yard drive begun after a short punt by Oklahoma State's Rich Thompson, kicking from his end zone.

Oklahoma's first score was a 27-yard kick by Lashar with 12:13 left in the second period.

Lashar's 30-yard field goal with 5:44 left in the game was his 11th in a row and a school-record 15th for the season, breaking Uwe von Schamann's mark of 14.

Oklahoma State was hampered by poor field position throughout, starting four drives at or inside the 10-yard line. Tailback Thurman Thomas picked his way over the ice for 100 yards on 23 carries, becoming the first rusher to gain 100 against the Sooners this year.

Oklahoma, 9-1 overall, completed its Big Eight season 7-0. Oklahoma State fell to 8-3, 4-3.

Oklahoma closes its regular season next Saturday against Southern Methodist, then faces top-ranked, unbeaten Penn State Jan. 1 in Miami. Oklahoma State is headed for the Gator Bowl Dec. 30 against Florida State.