Not seeing how major pruning by a contractor every so many years into perpetuity is an economical choice. And if you have turned them into pollards or otherwise severely pruned specimens then the charm of the natural shapes has already been lost - maybe plant some more suitable kinds of trees among them, cut the olive trees out after the new set is of some size. Otherwise they should probably be trimmed frequently, maintained as formal specimens instead of getting whacked back hard at longer intervals. Every time they are forced to regrow most of the top that was present at the time they are drawing on nutrient reserves in the trunks and may be gradually becoming diminished by the repeated assaults. There is also the possibility of trunk decay developing due to the large pruning cuts being made. For planting near swimming pools you want plants with large parts that do not get sucked past filters. As you have described olive trees are probably about as bad a choice as one could come up with.

While olives may be quite drought tolerant when established, they DO appreciate watering during very dry spells. It is suggested to water 1-2 times per month in summer, with a slow deep soaking (not a sprinkler) - no water in winter. A soaker hose or drip irrigation are very efficient methods of slow water delivery. Flowering and fruit production depends heavily on proper pruning practices. Production is heaviest on one year old wood and you want the tree canopy to be vase shaped, open and airy. There is an old Italian saying regarding olives that you want to prune them so that a "swallow can fly through them", meaning that light and air are allowed to penetrate into the center of the tree. This link offer good pruning tips and a helpful video. What is causing the dieback is impossible to determine from just the photos. If you resided in this country, I'd suggest taking in photos and samples to a plant pathology specialist (via extension service) for diagnosis but not sure what similar services you may have in Turkey. This link provides an excellent overview of olive care and cultural requirements plus outlines a number of disease pathogens - some quite serious - that can affect olives in a manner similar to what you are seeing. Regardless, any dead portions should be removed but until you are able to ascertain exactly what is causing the problem, I would suggest sterilizing any pruning equipment between cuts. But my first order of business would be to locate someone local to you to properly diagnose any possible disease issues.

Hi @Sunny Do, Thank you very much for sharing your photos and observation. I totally agree with you that the Osmanthus aurantiacus has very strong pleasing fragrance. Anyone who want to get one, right now is the best time to visit your local nurseries looking for them while they are flowering. (Osmanthus aurantiacus and other Autumn blooming varieties only bloom from late September to early November, but they have the strongest scent.) I imagine they would not stay on the shelves for long for such attractive smell. I would buy the ones which are flowering rather than those without flower. Many years ago, I bought my first young Osmanthus aurantiacus and other Autumn blooming varieties without any flower at the time. I waited for many years for them to mature to bloom. It was really a painful wait. But once the plants start blooming, they will bloom each year thereafter. Years of painful waiting memory will be forgotten. The Osmanthus aurantiacus took the longest wait.

Now you can buy from online, like Amazon. Also we created some faux black olive trees, not sure if we made it right, wanting to get some suggestions on the styles. here is the link: https://www.sinleenplants.com/black-olive-tree/