Leaf curling is a natural phenomenom in many plants to prevent excess transpiration from the leaf surfaces when there is a lack of water transport to the leaves.
In the olive tree, when the leaves tend to curl, as it is in your case, it is generally because the leaves are unable to obtain all the water it needs.
This can be a temporary situation as when there is excess water in the soil, or any other conditions that prevent the sap to circulate normally in the xylem vessels to the leaves, like root rot,
Verticillium wilt, girdling of the trunc etc....
Here is a link that describes a problem a bit similar to yours :
http://answers.google.com/answers/threadview?id=162898