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
It wouldn't explode, you need an oxidizer for that. But yeah, we would all dissolve into a pile of goo, since the lipid bilayer of the cells would lose their hydrophile component, probably leading to something similar to soapification, and even if that doesn't kill us, our DNA would degrade in an instant, since oxygen is the binding element between the phosphor atoms which hold the nucleotides together.