Why have my Olive Tree leaves turned brown?

Drought - you've not been watering enough. Give it a thorough soak by sitting it in a tray or something, and pouring water into it until its sitting in a pool, leave it for an hour. If all the water's gone, put some more into the pot and leave for half an hour. Then remove from the tray. Keep it well watered from then on - don't know how big the pot is, so can't say how much water to give it, but you should be watering so that the compost is spongy and soaked right through, every 3 days, and leave it to drain without a tray or anything.

I suspect Bamboo is right, but when you say 'I repotted it', into what size of pot did you repot it? Because the other possibility is that you have overwatered it and that the roots sitting in damp compost have rotted. It's a common problem for people to overpot small plants into pots far too large, especially slow growing things like olives.
You can find out by removing it from its pot and looking at the compost. If it's drought you will find the compost is bone dry apart from the top half inch or so (a common problem with soilless composts); if it's overwatering you will find the roots are sitting in damp compost and probably smell where they have rotted away.
Let us know what the problem was once you've done this!

Thank you both for your replies. I took the olive tree out of the pot and it was saturated with water. I had put stones in the bottom of the pot for drainage but obviously this had not worked. I have now repotted it back into its original pot with drainholes and hopefully it will florish! I'm new to gardening as you can probably guess!

Don't put yourself down, Att. Overwatering is a mistake we've all made, and not just once!
I hope your olive tree makes a good recovery.

Sounds as if the pot didn't have its own drainage - it is quite hard to overwater a potted plant, provided there are holes in the bottom of the pot which aren't blocked and its not sitting in a tray. Hope the plant recovers.

We have 60 Olive trees in our garden. They are never watered and often survive for months without water. They like free draining soil. Very often just one branch of a tree will die and we have always assumed that it is Nature's way of pruning the tree. The only thing that we have found which damaged the trees was the weight of snow on the branches when we had heavy snow here some years ago.
This year when we cut our trees back hard after olive picking we noticed that some of the trees are being eaten away by ants.
My advice would be to keep watering to a minimum.

If the olive is in a pot, though, Maggiski, then watering it more often will be necessary at certain times of year - a plant in the ground will seek its own water - in a pot, its trapped, and relies on human intervention to supply it. I'd also suggest you examine your olives for signs of aphid or other infestation - ants on plants usually means they're harvesting the honeydew from such an infestation.