Clark was a passenger in a car that was pulled over at around 10 a.m. local time for a seatbelt violation, when police determined there was marijuana in the vehicle.
As a backup to star point guard Marcus Smart, Clark was averaging 7.0 points and 3.7 assists in 16.6 minutes per game. He missed four games earlier this season for a suspension that ESPN reports was also related to marijuana.
Clark's arrest could give the Cowboys their second significant loss in as many days. Oklahoma State lost starting center Michael Cobbins for the season with a torn achilles on New Year's Eve.
The arrest also came just two days after Clark put in one of his best performances of the season. He scored nine points and had nine assists in the Cowboys' blowout win over Robert Morris on Dec. 30.
Oklahoma State opens Big 12 play on Saturday at Kansas State.