I believe there are too many variables to easily answer your quesiton. This would depend on the performance of the specific plug in systems - once they are developed of course.

Not much electricity comes from oil. The vast majority of electricity in the US comes from coal or natural gas. Hydro would be another big producer and then Nuclear. Oil fired electrical plants are rare.

The numbers I have seen on electric cars appear to indicate that Electrical cars will have the cost efficiency and the lower emmision performance when compared to the VERY inefficient internal combustion engine. Remember that even the best ICE's have very low efficiency.