I had considered chemical and air bubbling methods. However, I think electrolysis would be the best solution. It does occur to me that any physical changes to the oxygen concentrations may result in adverse effects after the fact.
If a chemical was to be used it would need to be dispersed at the right depth. Perhaps it could be encapsulated is such a way as it would be released in the right place at the right time. I am unaware of what chemical could be used and what methods of distribution would be effective.
As for bubbling air, this would be the least efficient, if for no other reason than all the energy lost during compression. Assuming it was somehow pumped down to those depths it would require a lot of pressure. The ideal gas law shows just how much energy would be lost given the pressure and temperature changes. Even if pure oxygen was delivered, it still needs to be refined and pressurized. All of this requires lots of energy.
As for the electrolysis idea, a sled fitted with an anode and cathode could be weighted to float at the appropriate depth and towed behind a ship. Electricity generated on the ship would be used to power it. I feel this would be the most direct, and perhaps easiest, way to deliver the needed oxygen to the depleted areas.
All ideas aside, scale is probably the most important factor. Even our best efforts may be essentially futile given the vast amount of oil and gas that needs to be metabolized.
Lastly, assuming any success with increasing oxygen concentration, there will be a bloom of microbes. This bloom will flourish so long as there is ample O2, but will quickly die after it uses it up. So, perhaps trying to “fix” a “dead zone” would in fact create a worse dead zone after the microbe bloom collapses.
OK, just let a nuclear sub troll the depths trailing the sled. I like it!
Clever thinking. We’d need an electrical engineer and an oceanographer to work it out, but it might work.