Oxygen absorbing caps are not necessary if you are using priming sugar to carbonate your beer in the bottles. Any oxygen remaining in the bottles after capping will be used up by the yeast when they re-awaken to ferment the priming sugar.
If you are using a counter pressure bottle filler or a beer gun to bottle beer that has been force carbed in a keg, and plan on having them sitting around for awhile, like say, for shipping to a competition or something, or long term storage, then the oxygen absorbing caps might be a good idea.
John