[MikeHardman]

open-centredness...
well, I can think of three reasons why it might be a useful idea:
- it would encourage development of lateral branches from the top of the trunk, which:
  - might be more prone to produce new growth (hence flowering/fruiting growth) than more-vertical branches
  - might provide easier/better supports for ladders while picking fruit
  - might provide a natural platform on which to stand, from which you would have easier access to the 'rim' of fruiting branches (imagine standing in the middle of a big cup)
Just theorizing.

[JTh]

I agree with Mike's comments about keeping the center open, it will give more light and enhance the fruiting, and it is easier to harvest. I find it quite simular to what I have been taught here in Norway abot pruning apple trees, the branches should be sufficiently open to allow a hat to be thrown between them. Well, I suppose the need for light is greater in the north, that's why a hat is used as a measure here, while a bird is more fitting in Greece.

[John]

It would also mean better ripening of fruit on any tree and some possible reduction is some cases of disease

[David Bracey]

Take a good look at an olive tree in full bearing.  The majority of fruits are found on the outside of the tree; they simply do not grow in the middle. Also the South West side of the tree bears more fruits than the rest of the tree at least that is the situation in the Gard. Olives are produced on one year old wood. 

Certainly pruning reflects picking practice.  It is far easier to pick from the ground than to climb a ladder, I know from experience.

Old olives were grown "the old way" ie in a square just like old Bramley apples and cherries.  Propagation was very difficult, until the arrival of mist, and there were few rooted cuttings to go round.  The square allowed horse or tractor cultivation.  The trees were allowed to "fill" the space.  To-day olives are grown in a hedgerow, with very dense plantings and the use of mechanical pruning. They "fill" the space within  a few years. Herbicides I presume are used in these intensively grown plantations for example in Northern Spain and I assume in Australia and California.   

While ancient olive trees maybe fantastic specimens they can tell a hard story.

[andrewsloan]

We have just harvested 780 kgs of our olives of the manzanilla variety with the help of some friends during 3 days. I had expected the yield to be lower after 240 litres of rain in October & November here in Malaga Province,Andalucia but I came back from the mill yesterday with 155 litres ,a yield of 19.6%. In other parts of Andalucia, like Jaen where there are a lot of olives, the harvest doesn't start until end December or early January, but around here it is warmer and the olives are ready earlier. In the photo one can see how green this year's oil is, compared to last year's, and a lovely fruity taste. It has 0.4% acidity.

[MikeHardman]

There is an excellent small collection of articles on olives on the MGS web site - http://www.mediterraneangardensociety.org/olives.html
Contributors: Brian Chatterton, Chevrel Traher, Dr. Peter A. Roussos, Davina Michaelides. Many thanks to them.
We have discussed pruning/shaping olives in this thread; peruse that link and more will be revealed.

[Alisdair]

You can find advice about this in the Fruit and Veg section: for a shortcut to get you there just click now on curing olives