Area of Triangles · Superchallenge problem

A trapezoid has a bottom side of $12$ cm, a top side of $4$ cm parallel to it, and a height of $6$ cm. Its two diagonals cross at a point $O$ that is $1.5$ cm below the top side. The diagonals cut the trapezoid into four triangles. What is the area of the triangle that has one of the slanted sides (legs) as a side and $O$ as a corner, in square centimeters?