Telomere length in these studies gets used as a proxy for "cells acting young" or "cells acting old." Telomere length tends to shorten as you age and it gets looked at as a proxy for de-aging your body at the cellular level.
Any simple treatment like oxygen therapy, will be unable to fix DNA damage. It has mutated already and thats that. What it might do is to "lie" to the cell. "You are perfectly healthy", when it actually isn't. This has benefits, such as more rapid healing of injuries, and it has drawbacks such as cancer.
We don't really have a good definition of "aging" because we don't really know what causes the body to give out after a certain amount of time has passed.
The factual, reality answer is clearing senescent cells.