The question in the book is about elemental composition of the given substance, in terms of mass.
You are given the empirical formula, i.e. what could be obtained by elemental analysis, namely what elements composed the substance and in which relative ratio. This does not tell you about the spatial arrangement of the various elements and the way they bound each other. The empirical formula can be even "smaller" than the actual chemical entity, all that is important is that for a given amount all elements will be always present in that ratio.
The fact that you read O3, it does not mean that you have ozone inside a molecule or salt, and the three oxygens might be at different sites within the structure (by the way, ozone is a polar molecule itself not an element or elemental ion, why it should be found combined? ).
While this should answer your current question, I invite you to reconsider the previous chapters of the same book, which almost certainly explains what we call elements and why, the laws of chemical combination, and the different levels of chemical formulas.
Now let's move to the reason for why I've choose to answer instead commenting "there is not ozone" or simply ignore your post.
Once the above is clear, consider that in cases the content of a substance (not its empirical formula) can be given in terms of a formal equivalent of another one. This is pretty common discussing or even classifying substances for a specific purpose or effect, so it might be with drugs, fertilisers, etc.
In your case, it could have been the amount of ozone evolving from the sample if a process would convert in ozone all the oxygen contained in it. Note that the amount as mass won't change as a certain amount of oxygen remains the same in all allotropes :) which again makes your doubt quite unmotivated.
Realistic examples of this formal way to express the chemical content by formal equivalents are fertilisers. Their N-P-K numbers refer to elemental nitrogen, phosphoric anhydride and potassium oxide, respectively. The latters are not the actual content of the fertiliser (as it wouldn't be ozone in your unfortunate example, too) but refer to the way as P and K were finally weighted in elemental analysis.
Another example is the carbon dioxide content of whatever fuel or combustible. There is not carbon dioxide itself in sugar, but its C content can be expressed by the carbon dioxide formed during its combustion.
Hope this helps.