Rate of reaction and dependence on concentrations
If we carefully see them ozone formation is a complex ( not elementary) reaction. 
Yes the slowest step would be rate determining .
so for slowest step ;

Rate =$   k [O] [O _3] $

where $k$ would be rate constant. 

but  final reaction is :
$$\ce { 2 O_3 -&gt; 3O_2} $$
and so ,
equillibrium constant K(for 1st reaction)

$K= \frac{[O][O_2]}{[O_3]}$

thus substituting $[O] $from above equation to the rate equation .

We get 

Rate =$\frac {  k × K [O _3]^2 }{[O_2]}$

And hence if $[O_2]$ is increased it causes rate to decrease.