This is an abstract from Christine's link above shown below.
As to quality compared to oil I suppose it depends how they keep, the impact and content of the brine they are stored in, and where stored in oil the type and quality of oil they are kept in.
And how olives keep appear to be a whole other subject as a quick search revealed!
http://public.carnet.hr/ftbrfd/40-129.pdf
The chart from the above link which suggests significant degredation in 15 days acidity double, peroxides up, phenols down with storage which MIGHT be applicable to olives in brine or oil.
My guess based on the above would be fine as a treat, but as a primary source good quality virgin oil, dark bottles etc looks a better bet, particularly given all the unknowns.
RB
ABSTRACT
"Calories per olive: It depends on how big the olives are and what their oil content is. The large black canned olives are actually quite low in oil - sometimes only 7%, which is why they are table olives and are not used generally to make oil. Some smaller olives used primarily for oil making can have up to 35% oil content. Olives can range from 1 to 14 grams in weight.
There is about 1 tablespoon of olive oil (and about 120 calories) in:
20 medium Mission olives that have an oil content of 20%
40 small ripe black olives
20 jumbo ripe black olives
7 super colossal ripe black olives"