Q.Why Is My Olive Tree Losing Leaves ?
I have a large olive tree in a pot outside that has been fine for several years. In the last couple of weeks it has started to lose lots of leaves. Some of the branches are now bare. I’m not sure if it’s under/ over watered. Some of the leaves have marks on them and are curling up. I have also noticed lots of ants crawling about in the soil and some on the trunk. I’m wondering if they are building in the soil and damaging the roots? I have started to feed with olive tree food but worried it’s going to die.

Certified GKH Gardening Expert
I think there are several factors, here. I do think that the ants are doing damage, but it shouldn't be that bad. What would cause this much decline would be that the tree is struggling to get water with such a cramped root space. This leads to situations like this where rot will take over. Unfortunately, a new container, roughing up the roots just a little, using a fungicide,and replanting into a larger container might be your best option. With such cramped root space the tree cannot get enough water and nutrients. You did the right thing by feeding, but there is still the issue of the cramped root system.
https://www.gardeningknowhow.com/plant-problems/environmental/root-bound-symptoms.htm
https://www.gardeningknowhow.com/edible/fruits/olive/growing-olive-trees-in-containers.htm