I am curious how and why this S60 AWD ended up solely RWD?
My research indicates that the final drive of this Volvo is primarily front wheel drive with a hydraulic actuated, multiplate clutch transfer for rear differential. The transfer case takes the drive off the front differential housing and sends power to the rear differential. The amount of power transfered is controlled by the traction control module and the difference between the input shaft speed (front wheel rotational speed) and the output shaft speed (rear wheel rotational speed) through that multiplate clutch. The Dynamic Stability Control also gets involved in the transfer as well as active differential wheel brake control.
I suspect that your oversteer condition has something to do with the lack of front wheel propulsion and the DSC getting confused because the global system was not designed for the conditions you are presenting to it i.e. rear wheel drive only. Also the mechanical roll control through antiroll bars would be designed for AWD propulsion.
I assume that you have checked the tire inflation pressures and they are spot on. I also assume that you have inspected the tires for overall wear; abnormal wear conditions; and circumference uniformity. It probably would be wise to have a four wheel alignment done as the "minor accident" might have done more damage.
Hope this is of interest to you.