Certified GKH Gardening Expert
There are so many factors: how big is the barrel and the soil volume in relation to the tree size; how long has it been in the barrel since potted/re-potted and the soil re-charged? A tiny barrel and a big tree of course is inadequate. Container planting needs re-potting every two years or so. A wood barrel is preferable to plastic. Plastic can retain too much water.
What is your watering program? Too much or too little could be a major issue. For olives, it's better to water too little than too much.
Feed the olive tree with nitrogen rich compost, conventional fertilizer or concentrated organic.
Check the trees cultural needs against your care: