What happens if you eat a jack-o'-lantern?

Toxic. Causes severe cramping, vomiting and diarrhoea beginning within a few hours, often severe enough for hospital treatment. Not usually fatal but genuinely unpleasant and dangerous to the frail.

Jack-o'-Lantern vs Oyster Mushroom

The test that settles it

Omphalotus is orange throughout and grows in fused clumps from a common base, not shelving tiers.

Read this before anything else

Never eat a wild mushroom identified by a website or an app.That includes this one. These tools narrow possibilities; they cannot see your mushroom, its base, its smell or its habitat, and the difference between a meal and a liver transplant is sometimes a feature buried in the soil.
Get it confirmed in person by an experienced local forager or a mycological society. Most regions have one, most run free identification sessions, and they will look at the actual specimen.
Dig, do not pull. The cup at the base of a death cap is often underground. Pulling the mushroom leaves the most important feature in the soil.
If someone has eaten a wild mushroom and feels unwell, call poison control now — in the US, 1-800-222-1222. Do not wait for symptoms to worsen. Amatoxin poisoning has a false recovery phase, and delayed treatment is what makes it fatal. Save any uneaten pieces for identification.

Jack-o'-Lantern

Toxic

Omphalotus olearius / illudens

Under the cap: gills
Spore print: white, cream
Growing on: wood dead, wood living
Stem: Orange, tapering toward the base, and fused with its neighbours in a dense clump.
If eaten: Toxic. Causes severe cramping, vomiting and diarrhoea beginning within a few hours, often severe enough for hospital treatment. Not usually fatal but genuinely unpleasant and dangerous to the frail.

Oyster Mushroom

No known toxins

Pleurotus ostreatus

Under the cap: gills
Spore print: white, cream
Growing on: wood dead, wood living
Stem: Very short, off-centre or absent, with the gills running right down it.
If eaten: No significant toxins and widely cultivated. The main lookalike concern is the toxic Omphalotus, which is orange and grows in fused clumps rather than shelves. Confirm locally.

Confirming jack-o'-lantern

True gills — thin, sharp-edged, blade-like, and separable from the cap flesh.
Grows in dense clumps from wood, stumps or buried roots. Chanterelles grow singly from soil.
Solid orange throughout, including the flesh.
Fresh gills sometimes glow faintly green in complete darkness, though this needs full dark adaptation to see.

Confirming oyster mushroom

Gills run down onto the very short, off-centre stem.
Growing in overlapping shelves on wood.
White to cream spore print.
Soft flesh with a faint aniseed smell in fresh specimens.

Still not certain?

Then you have your answer, and it is not to eat it. Run the full key, then take the specimen to someone who can hold it.

Identify it step by step