How to calculate therms usage from oil usage?
Figures, just found that 1 gallon of oil equals 1.35 therms. 
Another consideration is the efficiency of the unit. 
From a recent energy audit in know its efficiency is 79%.

This means my 100 gallon estimate is 135 therms. 
Now to try and guess what the gas bill would look like, based on their charge model is another story.