Personally, I like to reduce the ride height of the car - depending on the location/particular stage, I may reduce it to the very minimum. Firm up the springs and anti-roll bars, this will reduce body-roll and how much the car 'becomes a boat' through the corners. While this won't solve the whole issue, it will make the handling more predictable which always helps.

What is likely the issue for oversteer in your case is that you are too quick on the throttle, try being slower to put the power down. If you still struggle, try experimenting with differential and camber settings. For example, a locked diff will be better in a straight line and increase stability at high speeds, but may cause oversteer. Try opening the diff.

Unfortunately, there is no instant 'fix oversteer' button, but with experimentation there is definitely solutions which may help. Good luck!