None.... decomposition of aluminum oxide doesn't produce oxygen gas.
Are you (I suppose the question writer) assuming that aluminum oxide decomposes to form aluminum and oxygen? Think again. That doesn't happen. Aluminum oxide (aka alumina) which is made from bauxite goes through an expensive electrolytic process that produces aluminum metal and carbon dioxide (and carbon monoxide), but not oxygen. If it was easy to decompose Al2O3 into Al and O2, don't you think the aluminum industry would be using it?
The naturally formed mineral corundum (ruby and sapphire) is aluminum oxide. Luckily for the gemstone industry it doesn't decompose to form aluminum and oxygen.
Rather than attempt to answer this question, you should take it back to your teacher and ask them to write a question that is realistic.
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DCM5150 as added a comment referring to a video showing the decomposition of a tiny drop of molten alumina decomposing at over 2000C. Interesting, but not a practical way to decompose Al2O3. While it does produce O2 gas it doesn't produce aluminum metal. Instead, it produces an alloy of aluminum and iridium (a rare and expensive metal), rather than pure aluminum metal. Iridium was used as an electrode. And while this is an interesting development, it would not be practical to produce over 5 moles of O2 gas, nor will it produce aluminum metal.