Hello!

It needs feeding, but this should normally be done in spring. Check out this link for more information:

https://www.rhs.org.uk/advice/profile?pid=138

Lovely tree - I do hope you can cure its problem. (Yellow leaves are usually a sign that the plant is too dry.)

The picture indicates that although you have leaf drop, there doesn't appear to be any dieback. I don't think that lack of water would be a problem as they thrive in dry soil. Evergreen trees do lose there leaves so can you see any new buds with leaves emerging? They also thrive in poor soil so fertilizer should not be a problem. The virus Xylella fastidiosa will not have reached Japan so it wouldn't be that. I would leave things as they are.

First, dig gently around the trunk, to see if it was planted too deep. The topmost major roots should be just barely underground.
Next, what is the drainage like there? If there is bedrock, heavy clay, or a hardpan layer underneath the tree, the roots may be damaged.
Also, what kind of feeding have you been doing, and how often--Olives don't need a lot of food, but they still need some, especially if this is one of the warmer parts of Japan.
When you water, how do you do that? When and where watering is necessary, Olives like deep, infrequent soakings. The water should penetrate at least 70 cm into the soil, and, maybe, once every 6-8 weeks in Japan.

Thanks everyone for the advice. I dug gently around the trunk and found a fairly thick root about 2 cm below the surface so I don't think the tree is planted too deep. With regards drainage, the sandy soil and mulch mix goes down about 1 meter, beneath which there is hard, compacted soil, possibly clay (I remember it being extremely hard to dig out which is why I wanted to have at least a meter of soft soil above it). Do you think the hard soil might be acting as a barrier to prevent water draining away? I assumed lack of water was the problem rather than the other way round because the leaves look very dry, but I imagine water logged roots could have the same effect by preventing moisture being transported to the leaves. If there is poor drainage this would have been compounded by planting the tree just before the rainy season during which it rained a lot for about six weeks. After that I watered the tree about once a week with a watering can, giving it about 12 liters of water each time as recommended by the gardener because the summer months get very hot here with maximum temperatures close to 40 degrees Celsius. As for feeding, the original soil mix contained mulch and some pellets of slow release fertilizer. This Spring I spread a new layer of mulch over the top soil and added some more pellets of slow release fertilizer just below the surface but away from the trunk. I should also add that I sprayed the tree with a pesticide last summer and this spring. The pesticide was a standard pre-mixed type purchased at a garden center so in theory it should not have damaged the tree. Interestingly, although the leaves are in poor condition there are quite a few olives on the branches and they look very healthy.

First, it is planted slightly too deep, but possibly not enough to worry about.
As for the drainage, the picture that I am getting is this: you have heavy clay soil, so you dug a big hole, and replaced the soil with "topsoil" mixed with mulch. There are a couple of things wrong with that:
Any sudden change in soil texture will slow or stop drainage, so, where possible, it is better to amend the native soil than to replace it in the planting hole. Did you do a percolation test? Drainage of at least 2 cm per hour will allow you to plant directly in the native soil, with amending with organic matter--preferably with compost, rather than "mulch". Slower perc rates will mean planting trees and large shrubs in meter tall raised beds, with weep holes near the base, and possibly perforated drain pipe installed, if the bed is more than 2 meters wide.
Often, bagged products--or sometimes even bulk--sold as "topsoil" are actually a heavy potting compost made with composted fine bark. When that is buried for long periods, it tends to start to ferment, especially in high temperatures, producing toxic gasses that kill roots.
Watering might be an issue, too. Even if the drainage is good, in 40º temps, an olive that size will need around 70 to 90 liters of water, about 2 times a week, since it is essentially planted in a big pot. Here, in drastically lower humidity, we need to water the olives in containers every 1-2 days.

Just a quick update. I bought a probe and tested the PH of the sandy soil mix. The result was 5.0 which I think is a little too acidic for olive trees. Do you think this might be at least part of the problem? I have another olive tree that was planted directly in the native soil and that one is positively thriving even though it hasn't been watered or fertilized at all.

Yes, much too acidic for olive trees, Outernation. They like a neutral to slightly alkaline soil. You might try drilling holes in the ground with a soil auger, about 30 cm apart under the drip line. Mix the soil from each hole with about 5 ml of coarse ground limestone, preferably dolomitic, before sweeping it back in. That will provide areas of high pH where the tree can pick up nutrients. Switching the bark mulch out for limestone or marble gravel might also help.

Thanks Tugbrethil. I'm actually thinking of starting over again with new soil and a new tree. Would you recommend replacing the current soil with native soil from another area of the garden or replacing it with a commercially available soil mix? Also, do you think it would be okay to plant now in the autumn, or should I wait for spring? The climate here is mild with temperatures only occasionally dropping below zero over the winter.

Unless the tree has strong sentimental value, replacing it is probably the smartest way. I would replace the present mix with native soil lightly conditioned with compost--it should make life so much easier.

Thanks for the advice. Would you recommend planting the new tree now or waiting until next spring?

Where I live, fall is the best planting season, but we only get a few days of frost each winter. If you get a lot more than that,you might want to wait until spring.

here is a very useful link:
http://www.haifa-group.com/knowledge_center/crop_guides/olive_tree/

http://www.haifa-group.com/knowledge_center/crop_guides/olive_tree/mineral_nutrition_of_olive_trees/#{B759282B-52D3-447E-898A-B8A67487BDBD}
i hope it solves your problem...

Sorry, I tried to comment after trying to use an incomplete web address. The actual site is pretty useful, though I suspect more is going on than just a nutrient deficiency.