The aim of this work was to study the effects of agronomic application of olive mill wastewater (OMW) in a field of olive trees on olive fruit and olive oil quality. Agronomic application of OMW increased significantly the fungal:bacteria ratio, whereas the root colonisation and the photosynthetic rates decreased significantly. Consequently, the oil content expressed as a percentage of dry weight, decreased significantly after agronomic application of OMW. Land spreading of OMW altered the relative proportion of individual olive fruit sugar and decreased significantly the nitrogen (N) and phosphorus (P) of the fruit. A significant increase was observed in total phenol content of oil after agronomic application of OMW. alpha-Tocopherol content, on the contrary, decreased with OMW application. The fatty acid composition of the oil was not affected by the treatments. To our knowledge, this is the first report of change in the olive fruit and olive oil quality following agronomic application of OMW.