FormerlyHemlock
Hero
It can destroy any civilization that doesn't have magic, since it's entirely immune to non-magical attacks. It could destroy the modern US, for example, though it would take a while.
Technically this isn't quite true, since for reasons that make no sense to me (and therefore don't count when I'm DMing), Crawford has ruled that non-magical weapon immunity doesn't protect you from falling damage. http://www.sageadvice.eu/2015/08/31/immunity-to-bludgeoning-does-take-damage-from-falling/ So the modern U.S. military just has to trick the Tarrasque onto... a mountainside, or the Grand Canyon, after it's rigged with explosives. Any given fall will only cause 20d6 damage, so you have to make sure the fall is broken up into many little falls of around 200' each. Falling a total of 2000' or so should kill it.
Also, there's thunder/lightning damage. (What damage type should an Oxygen Destroyer do? Force?)