What is the oxidation number of carbon in carbon suboxide?

$$\begin{gathered} +2 0 +2 \\ \mathrm{O}=\mathrm{C}=\mathrm{C}=\mathrm{C}=\mathrm{O} \end{gathered}$$

In ${\mathrm{C}}_3{\mathrm{O}}_2$, twe C-atoms linked with oxygen atoms are present in +2 oxidation state and central carbon has zero oxidation state.
So, the average oxidation state of carbon is $+\frac{4}{3}$.