How is the 'rate law' and 'order' for the ozone decomposition reaction can be defined?
Chemical kinetic describes both reversible and irreversible reactions. 

The reaction scheme and rate expression do not refer to the same thing. The reaction scheme is an overall one whereas the rate equation refers to a particular multi-step reaction. Two examples are given below:

Ozone can be destroyed by UV light in the $240$ to $\pu{320 nm}$ region with the scheme $(\ce{M}$ is an inert gas needed to take away excess energy):

$$
\begin{align}
\ce{O3 &amp;-&gt;[$hν$] O2 + O} \\
\ce{O3 + O + M &amp;-&gt; O2 + O2 + M} \\
\hline
\ce{2 O3 &amp;-&gt; 3 O2}
\end{align}
$$

Similarly, oxygen molecules can react with short wavelength UV as

$$
\begin{align}
\ce{O2 &amp;-&gt;[$hν$][$(&lt;\pu{242 nm})$] O + O} &amp; \\
\ce{O + O2 + M &amp;-&gt; O3 + M} &amp;|\cdot 2 \\
\hline
\ce{3 O2 &amp;-&gt; 2 O3}
\end{align}
$$

which also gives a similar net reaction. This is the Chapman mechanism and is very important in atmospheric chemistry.