From what I understand, Dissolved Oxygen is nothing but Molecular Oxygen (O2) loosely bonded with water -- and thus the bond is not chemical, and breaking it would not result to any changes in the chemical composition.
The maximum amount (%) of oxygen water may dissolve under constant temperature and pressure is almost fixed -- proportional to the solubility constant of Oxygen in water.
Usually, the solubility should decrease as the temperature increases, as the molecules will have more energy to break loose bonds like hydrogen bonds (governed by Arrhenius equation I think).
This extra oxygen would thus be freed and go back to the atmosphere.
Onward to the question,
We know that oceans contain a good amount of dissolved oxygen. If we start gradually increasing the temperature (upto the boiling point of salty water), this would liberate more and more of the dissolved oxygen back to the atmosphere. Question is, what will be the % increase in Earth's atmospheric oxygen level if we were to start boiling oceans. Will it scale linearly (or following some monotonically increasing curve) with the temperature of the oceans, or will it hit a maximum and degrade, and what could be that maximum?
We further have two cases- (1) Assume that the aquatic plants/ creatures still keep functioning like normal even in boiling water ; (2) Assume what would happen in reality -- they would die.
Addendum: Let's assume we have high alien-quality nuclear fusion tech to cater for our energy needs. Also we can, for now, forget about the side effects, even if the world is damned.