I did a yahoo search for "can cats have olive oil" and this came up under "10 uses for olive oil":

#4:
Hairball Solution

This one is particularly important for cat owners. Hairballs are a common problem in cats. Just add a couple teaspoons of olive oil into a can of your cat’s food and your furry family members will be hairball free in no time.

I don't know that I'd add a couple of teaspoons, that seems like a lot.

Maybe wait for more replies before doing it - I've never done it so would wait and find out if anyone has first.