This is usually lack of or too much water.Both will cause leaves to droop. Also, was it bought from inside or was it out of doors? Could be it wasn't hardened off?

I just looked up Oleander on Wikipaedia. Depends on the variety you have . Some of them are poisonous, but they do mention ' drought tolerant.' Maybe thats the cause of your trouble. Too much water. Plants that grow in the Middle East would probably need less water.

I am not an expert, but remember doing an embroidery of a sprig off a shrub at the Lido de Jesolo, near Venice.
They had pink flowers, were evergreen, growing all along the roadway on the campsite. Not scented.
They were not watered at all. The ground was as dry
as a bone.

They're used here along the central reservation of motorways, so they get pollution as well as only water when it rains.

In containers, and when first planted, Oleanders need frequent watering--their drought tolerance is mainly due to their wide, deep, and aggressive root systems, which take time to develop. If it has been some time since signifigant rain, or a good soaking with a hose, that may be what it needs. If it doesn't perk up after a good soaking, double check how deeply it has been planted, and remove the excess soil if the topmost roots are more than 1/2 cm underground.