Fresh from a tree the green olives are very bitter. They need to be cured in a salt brine to be edible, and then they are quite hard to digest, unless you chew them very well. Almost all retail olives are very poor quality, seem to be cooked and very salty and mushy. If you are able to buy a crate of raw green olives when they are in season, you can try making your own cured olives, with basically salt and water and some pressure. There are also dried ripe black olives, but that is another taste experience.