I'm not an analytical chemist, but here's a suggestion.  The classical test for oxidation substances is starch color change, but I believe that it works the wrong way.  I believe that the test is similar to the one used in the BOD test, and standard methods might be able to provide some guidance.  I would also suggest measurement of the ORP.

Beyond that, I got out my chemistry book and looked, and here's a thought.  Try adding a starch solution and then titrating with a known strength of Hydrogen sulfide or some other powerful reducing compound.  The compound may start out clear and when the oxidant is removed, it will turn blue, At least I think that's what will happen.

Alternatively, try titrating with an solution of ferrous iron. It will react with the oxygen and the solution will go from blue green to rusty orange. The only problem is that I don't know how fast the reactions are.  Beyond that, I'm pretty much in the dark.