Ozone is created when ultraviolet light strikes oxygen and breaks it apart into two oxygen radicles, which in turn combine with other oxygen molecules to form ozone.

When two ozone molecules encounter each-other, an oxygen from each will bind to the other to make three oxygen molecules, plus heat. More ultraviolet itself can also destroy ozone, for obvious reasons. Therefore, ozone cannot become common because it destroys itself. The exact amount of ozone formed will therefore depend on the equilibrium between the ozone being created by ultraviolet light and ozone being destroyed. This happens where the atmosphere becomes thick enough that ultraviolet light cannot penetrate without encountering oxygen molecules, and it so happens that this occurs mainly at the stratopause - the boundary between the stratosphere and mesosphere.

I don't know for a fact what the proportions of gasses in the stratosphere are, but ozone will be more common there, and become increasingly common as you near the stratopause. I don't know what the maximum concentration is. I can also tell you that water vapor - the third most common gas in the troposphere - is almost nonexistent in the stratosphere (which is a good thing, because water vapor is extremely destructive to ozone if the two come together).

It's frustrating, isn't it?! I think this is the best answer, that ozone is a minor gas in the stratosphere, and the main gases are just the ones in the troposphere but in much lower quantities - a thousand times thinner than at sea level, according to the 2nd link:

"The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains high concentration of ozone in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere." From Wikipedia

Here's another link: stratosphere overview.