How Do Earthworms Get Oxygen?

The question may be assuming that earthworms live in air-tight spaces underneath the soil surface.  Actually, it's not airtight down there where they live.  Earthworms can really only burrow and live in soil that is relatively loose -- not densely compacted.

So they get enough air from within the tiny air pockets of the soil they live in.

As to how the oxygen gets into their body... earthworms don't have lungs.  But there is gas exchange across their skin, and into their primitive circulatory system (blood stream).  The outermost layer of the Earthworm's skin is kept moist by constant small excretions of mucous.  This wetness attracts (in a chemical way) oxygen into the worm, and helps to expel waste gases out of the worm.  Earthworms need to stay about as moist as a damp (not dripping) sponge to breathe well enough.

If the air pockets in the subsurface soil get filled with water (as they do after heavy rain) then the worm is forced to go to the surface to breathe.  That's why it's easy to find earthworms on the lawn after it's been raining.

We know that the earthworms live underneath the soil surface but the physics of earth is design by GOD in such a manner that there are always interstitial spaces present beneath the earth's surface in which water and air continuously diffuses by which animals like earthworms live. They utilize the oxygen from the water and diffusing air through their moist skin and they perform respiration..............

They get it from the soil.Soil has tiny spaces where they are air for earthworms to breathe.