Where are you located, nearest big city?

I love olives! Yum...

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We have Olives here growing in our decomposing granite soil or fill or whatever, Bart. They seemed to benefit from irrigation here. I agree with the keeping your plant in a pot while small with your conditions. I think we both have baking hot dry summers, but I am thrilled for you having more winter rain there in Italy than we get in So Oregon.

I'm not sure soil type matters as much as water. Here, my figs are blah and not producing in my toughest spots, and prefer better conditions in my chicken coop area. I think of Olives growing in the driest worst conditions, but that is not what happened here.

My olives are maybe 15 ft by now in 15 years old. I've been here 9 years and they were planted by our home builders who were Italian.

Bart, this may not be the best, but it's what I'd do: repot the tree keeping the clay, but crumbling it around the edges, not disturbing the roots too much, and blending the freed up clay with potting soil. Mind you, I'm not great with potted plants. If your clay's like ours, it needs diluting. But the clay added to the potting soil will help hold water.