Stephen Wolstenholme writes
Black olives do not need to be fermented and are often are only cured
but Green olives take both treatments to get rid of the bitter taste.
Years ago in Greece I sampled fresh olives, picked the previous day,
unfermented, uncured apart from some dry salt and they didn't taste
bitter. There are some varieties of olives that are not bitter and can
be eaten fresh off the tree.

Last year our drinks were being served with a plate of green olives,
picked from the owner's mother's tree and kept in brine for a few months
- at least that's what we understood from a combination of hesitant
english on his part and infinitely worse greek on ours. (I am sure about
the tree, though - we were taken to see it!)
--
Kay