Oxygen production from thermal decomposition of metal oxides
A lot of metal oxides decompose under liberation of $\ce{O2}$ when subjected to sufficiently high temperatures. For example, $\ce{HgO}$ decomposes at temperatures above $\pu{500^\circ C}$ into elemental mercury and oxygen:

$$\ce{2HgO -&gt;~ 2Hg + O2}$$

However, disproportionation is another possible reaction pathway. An example is the thermal decomposition of iron(II) oxide to elemental iron and iron(II, III) oxide at temperatures below $\pu{575^\circ C}$ ($\ce{FeO}$ is more stable above that temperature; sources: 1, 2):

$$\ce{4FeO \rightleftharpoons Fe + Fe3O4}$$

Another example is the decompostion of $\ce{SnO}$ at temperatures between $700\mathrm -\pu{1000K}$, which proceeds via an intermediate $\ce{Sn3O4}$ mixed oxide. Tin disproportionates from the oxidation state +II in $\ce{SnO}$ to $0$ in $\ce{Sn}$ and +IV in $\ce{SnO2}$ (1, 3).

\begin{aligned}
\ce{4SnO -&gt;~&amp; Sn3O4 + Sn} \\
\ce{Sn3O4 -&gt;~&amp; 2SnO2 + Sn}
\end{aligned}