Assuming that everything in the suspension is tight, oversteer is caused by the "slip angle" of the rear tires changing faster than the front.

Typical causes are low rear tire pressure, hard rear shocks, hard rear springs, hard rear anti-sway bar (all compared to the front), insufficient toe-in on the front tires, to much weight in the rear, or even different rear tire construction.

But first check the rear suspension and wheel bearings to make sure that something isn't loose.

(edited to correct grevious error)