All concrete collapsing, massive drops/increases in pressure that can shatter your eardrums, untreated metals cold welding, full UV rays going through an Ozone layer that no longer exists, planes fall out of the sky… oh and yeah the Earths mantle just fucking collapsing in on itself and essentially dissolving as its composition has an enormous amount of oxygen
If we’re only talking about oxygen as in O₂, you’d probably be mostly fine. Even if we’re talking anything that contains an oxygen atom getting destroyed, something like Silicon would be so much worse - most of the Earth’s crust would just be gone. Hydrogen would probably also be fun, because it’d turn water and any organic compound into oxidizers.