Forming half equations
$\ce{SrO}$ is an ionic compound comprising $\ce{Sr^2+}$ ion and $\ce{O^2-}$ ion.

The oxidation state of $\ce{Sr}$ changed from $0$ to $+2$, and $\ce{O}$ from $0$ to $-2$.

Therefore, $\ce{Sr}$ is oxidized while $\ce{O2}$ is reduced.



The ionic equation is just that, since there is no individual ion in the equation ($\ce{SrO}$ is a solid!).



Are you really told to find a half equation for a reaction not performed underwater?