The oxidation number of Cr in CrO₅ is

In CrO5, there are two peroxide linkages, which means that four O has -1 oxidation state and only one oxygen atom has -2 oxidation state.

$$\begin{gathered} \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}+4(-1)+(-2)=0 \\ \mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr}=+6 \end{gathered}$$