Is the ozone layer in the upper or lower stratosphere?

The layers of the atmosphere are differentiated by their gaseous composition, but were originally identified by the change in temperature. The stratosphere is the region above the tropopause where it starts to get warmer as you move upwards through the atmospheric column. This phenomena (warming with increasing height) is attributable to the presence of ozone, so the whole stratosphere is the ozone layer. The most ozone in found at about 20-25 km (it varies with latitude), as can be seen in the linked figures.