Dry yeast
needs NO oxygen. None. But oxygen won't hurt it at all.
You can work up dry yeast the same as you would liquid yeast. But then it does need oxygen when pitched, no worries about 'food' stores, wort is food. Fire up 1.5 L of 1.040 wort with a few pellets, boil, cool to 68F, pitch packet, pitch into full batch while still krausening... . Something like that. Just start it in the morning before you start brewing, a few hours will get it rocking like nuts since it's approaching enough cells for a standard gravity 19L batch. Be sure to aerate the wort. or add olive oil
Dried yeast is safe to repitch a couple times or so, I take it 3 generations sometimes, after that you're starting to play with fire unless you wash the yeast with ClO2. Dried yeast isn't pure, it does have minute quantities of other bacteria from the process of dehydrating it, these will multiply with each fermentation and eventually get you where it hurts. Liquid yeast are called pure cultures because they are, well, pure cultures.
Yeast does change after a few generations, usually reaches peak performance by generation 3, 4 tops, although gen 2 is usually pretty damn good. The difference is more pronounced with dry yeast. Yeast conditions itself to it's surroundings, namely wort. The yeast will ferment stronger, cleaner, more complete attenuation... yadda yadda... it will begin to mutate eventually, depending on the strain, flavours change, floc characteristics change...