Are there chemicals that absorb and release upon heating oxygen more efficiently than potassium oxide?
If you are looking for something that can be made using elemental  oxygen as an oxidizer and reversibly release the excess oxygen below 1000 °C, sodium peroxide beats potassium peroxide by using lighter metal atoms.  It can be prepared by reacting sodium with oxygen in two stages (forming sodium oxide in the intermediate stage) at 130–200 °C and releases the excess oxygen at 657 °C (Wikipedia).

Lithium, magnesium and of course hydrogen form peroxides with even higher excess oxygen percentages, but these are not made via reversible reactions with elemental oxygen.