Here we go again.
Driving other cars / 3rd party extension : Insures the policy-holder to drive cars other than that identified on the policy with 3rd party cover. The insured's policy is the covering insurance. Restrictions may apply e.g. engine size, value, car cannot be owned by the insured, etc
Open driving cover : Insures drivers other than the insured or those named on his/her policy to drive the insured's car. The insured's policy is the covering insurance. Restrictions usually apply e.g. drivers between 25 & 70, full licence, etc.