How many national championships has Oklahoma State won?

In Div.1a football....none.

Answer

According to the NCAA Oklahoma State has won 49 national championships as of the Spring of 2010. This is the 4th highest in the NCAA following UCLA, Stanford, and Southern California. Arkansas follows Oklahoma State with 41 wins. The University of Texas holds 2nd place in the Big XII with 39 wins.

Most of OSU's wins have been in Wrestling and Golf.

They have won championships in Basketball: 2

The answer above is correct, they haven't won any football championships.