Formation of strontium peroxide
There are a few method to synthesize $\ce{SrO2}$ from $\ce{SrO}$ and $\ce{O2}$. One of them uses $\ce{KClO3}$ as the source of $\ce{O2}$ [Ref. 1]. The paper is in German but has English abstract which states:


  Abstract. Single crystals of $\ce{SrO2}$ have been oblained after high
  pressure/high temperature reaction of a $\ce{SrO}$/$\ce{KClO3}$ mtxture at $\pu{20 kbar}$, $\pu{1400 ^\circ C}$. ...  From a refinement of the site occupation factor for oxygen a composition $\ce{SrO_{1.95(2)}}$ has been found for the crystal investigated. 


Note that $\pu{20 kbar}$ pressure was inserted by the $\ce{O2}$ released by the following reaction at high temperature (Ref. 2): $\ce{2KClO3(s) -&gt; 2KCl(s) + 3O2(g)}$.

Experimental information on the $\ce{Sr-O}$ has been reviewed in Ref. 2 and stability of $\ce{SrO2}$ and phase-diagram of $\ce{Sr-O}$ is graphically presented (see Figure 1 &amp; 2, respectively). Accordingly, $\ce{SrO}$ under $\pu{0.5 atm}$ of $\ce{O2}$ pressure at $\pu{800 ^\circ C}$ would not produce $\ce{SrO2}$ (see Figure 1: Only $\ce{SrO}$ exists under the conditions, $\log_{10} p = \log_{10} 0.5 = -0.30$ and $\frac{1000}{\mathrm{T}} = \frac{1000}{1073.15} = 0.93$).



References:


K.-J. Kange, F. Rau, U. SchieBl, U. Klement, Verfeinerung der Kristallstruktur von $\ce{SrO2}$ (Refinement of the Crystal Structure of $\ce{SrO2}$), Z. anorg. allg. Chem., 1994, 620, 879-881 (https://onlinelibrary.wiley.com/doi/pdf/10.1002/zaac.19946200521). 
D. Risold, B. Hallstedt, L. J. Gauckler, The strontium-oxygen system, Calphad, 1996, 20(3), 353-361 (https://doi.org/10.1016/S0364-5916(96)00037-5).