5G Oxygen is a targeted chemoweapon and the successor to 5G Corona, designed at some point in the 2050s to attempt to eradicate the Confederates once and for all after they were able to cure the replication crisis, brought on by necessity after they were replaced by the Buckt app in the Marvellous Chocolate Factory.
5G Oxygen has a similar chemical composition to normal oxygen, but the gaseous structure is in the 5G allotrope which oscillates at the oxygen frequency. Such an allotrope is highly toxic to the Confederates, whose immortal and invulnerable metabolism cannot handle the 5G Oxygen, but generally safe for humans - still, around 0.6% of humans suffered severe damage to their conceptual structures as a result of 5G Oxygen.
The atmosphere of the Earth now being toxic to the Confederates, they fled deeper underground, until they made contact with Pellucidaria, the continent of the Ediacarans.