Experiment in which I put several iron nails in two flasks with tap water. I heat one of the until the water boils and left untouched the other. After that I put a layer of oil on the water. One day after, we see the nails of the untouched flaks are rusted and the ones from the boiled water are intact. This is because when we boil the water we eliminate a lot of air disolved in it, so we remove most of the oxygen. The oil acts as a barrier to avoid air enter and get disolved into the water.

That is whay if we put a fish in a bottle (to transpor it) it is not a good idea to close it with a stopper or cap. If it is mandatory to close the bottle, then it is better not to fill the bottle full, but do it to the half so it is oxygen that pass to the water as the fish consumes it.

https://en.wikipedia.org/wiki/Oxygen
https://en.wikipedia.org/wiki/Water
http://chemwiki.ucdavis.edu/Core/Phys...
http://www.cienciabit.com