This picture is from our local town Hastings City, they regularly trim all the olive trees for this beautiful manicured effect.
The time of year you prune affects the growth result from the prune. The growth result from the prune affects the volume of fruit produced by the tree.
Always start your prune with the end goal in sight. Define your goal as "What do you want from your tree in 3 years time". That is: Where do you want the new growth to be on the tree?
If you prune the tree in winter it will not be growing much so you will get a lower growth response from the tree.
If you prune your tree in October - November the tree will be actively growing in its spring phase, so your tree will get a fast and heavy growth response.
So for Telegraph Hill we prune in October / November so that we get maximum growth response from the tree. We want this so that we maximise the new wood growing, so that in three years we maximise the fruit growing on this new wood.
Pruning Fact: Olive fruit will only grow on the wood that the tree produced last year. So pruning to stimulate new wood is the Telegraph Hill objective.
If you olive tree is in your back yard then you may have the opposite objective, so that the tree just looks beautiful and does not produce much fruit to stop the mess from the fruit (and from the birds eating them).