What difference does oxygen content of tea water make? 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?
All moving water has dissolved oxygen in it. That is what fish breath

Dissolved oxygen is reactive, and will most likely extract more substances from the tea leaf, than without it. If these are the good flavour parts of tea, I do not know?

When you heat water it starts to release the dissolved oxygen. The more you heat water the more oxygen escapes

You can buy tea making kettles that bring water up to 95°C (203°F), but not boiling, so as to decrease the amount of dissolved oxygen lost, but still making the water hot enough to brew tea. They also save energy :-) I use one of these, and am happy with it

Example Kettle

Some people "watch" their kettle, and switch it off just before the water boils!

It is a personal taste preference if tea tastes better when brewed in water with more dissolved oxygen or not