I bought a small olive tree from a local store last spring. I put it into more of a bucket than the vertical planter they gave me. Perhaps the container, little larger that a very tall glass, was shaped to control the shape and size of the tree. I transferred it to a much wider pot now, bucket shaped with side handles so that it can be brought indoors in the winter. It is maybe about a meter tall in all, now.
They said to bring it in doors. I should check to see how much cold an olive tree can take. Most of the leaved trees outside drop all their leaves but survive the -21C, -20F winters. Sometimes they say that a plant must be actually planted in the ground to survive that, that it needs roots below the freeze depth of the soil, and a potted plant can't do that.
Just like you were all saying though, cool and too much water. I have zero drainage in it and a light coating of sand on the top to pass the water down and in but keep it in, too. The difficulty I think is having it next to plants that DO like the more water even when it gets cool. Next to bamboo which loves water, too much maybe. I have mine in the sun of a tiny dinning room, south facing, in southern Michigan, USA. The houses are pretty close together, maybe four meters apart, so the low winter sun only gets inside in the afternoon. In the summer I often put an awning up to cut down the sun heat indoors.
So Thanks for the info!