Watering an olive is extremely important. There should always be sufficient water. Moisty but not soaking wet. Therefore you must give them regularly water. Specially during the spring and summer (until September). You can check it with your finger. When you place you finger in the ground until the second phalanx only the top of you finger should be wet the rest should be dry. (not moist but dry)
In Spain the roots of olive trees can grow up to 6 meters into the ground.
Even during winter time (when it does not freeze) an olive still needs little water. Rainwater is usually more than enough for the olive tree. We give our trees every three to four weeks a little amount of water. (During winter time).
The water must be able to escape from the pot thats why holes should be made at the bottom of the pot.
If the soil is too wet then one can decide to place the tree in a sunny spot just under a canopy or the clod partially covered with a board. The moisture content of the root ball should be between 20 and 30 %.