I've heard the advice that water should be boiled for tea only one--that when boiled it loses dissolved oxygen, and if there is insufficient oxygen in the water, the flavor of the tea is (somehow) affected.   This doesn't seem to make sense to me. If the water loses oxygen when it is boiled, it would have lost it before tea ever touched it anyway.  Does this really make a difference, or is it just a commonly perpetuated kitchen myth? If there is one, what affect does the oxygen (or lack) have on the finished product? 
This blog article (citing numerous sources) claims that re-boiling water doesn't have any significant effect on tea taste. Here are some key points from it:   Heating water above 50˚C already removes most of the oxygen from it, so neither once-boiled nor twice-boiled water contain significant amounts of oxygen. Triangle tests such as this one prove that dissolved oxygen by itself doesn't affect water taste. While oxygen could theoretically reduce tannin's concentration in tea, this effect is dwarfed by other factors, notably steeping time, water temperature, and water/tea ratio.      All this is not to say that water is unimportant. Water is important. Alkalinity is important. Salt content is important. Minimal iron content is super important. Dissolved oxygen is not important.   [...] For brewing tea, coffee, or any other hot beverage, dissolved gases are irrelevant.  