Ooma

"Passive outer being protecting an aggressive, explosive core. Ghastly, eerie creature made out of thick liquid. You can peer right through them, and can't see where they keep their thoughts. I don't trust them at all. Keep away."

Oomas are enemies in Hollow Knight. They have a pulsing orange core which can propel itself through the air if the outer shell is damaged. Monomon the Teacher has an unknown connection to them.

Behaviour and Tactics Floats up and down in place in a set pattern.

Its body bursts when hit, revealing its volatile orange core. After an extremely brief moment, the core homes in on the Knight, exploding on contact with the Knight, a wall, or an attack and dealing 2 Masks of damage to the Knight and 9999 damage to enemies. Ooma explosions only deal 50-150 damage to Uumuu when fought in Godhome.

The core is tricky to avoid due to its travel speed, so it is advised to mainly avoid attacking Ooma when possible; they are otherwise passive.

A direct hit from a Vengeful Spirit/Shade Soul projectile continues pushing the explosive core along its path, often until it hits a wall and explodes. This is the safest way to destroy an Ooma but requires SOUL to be spent.

Another way to destroy Oomas without dealing with the exploding core is acid. If an Ooma is pushed into acid (via a Nail strike from above), the core is harmlessly destroyed along with the rest of the Ooma. Unfortunately, this method of killing does not allow Geo to be collected from the kill.

The Weaversong,  Dreamshield, Glowing Womb,  Grimmchild, and Defender's Crest Charms attack them, and the volatile core is still launched at the Knight, so it is not recommended to wear these Charms while in Fog Canyon.

Location They are found in large numbers in almost every area of Fog Canyon.

Trivia
 * Shadow Dashing through an Ooma while using the Sharp Shadow Charm causes its explosive core to always go to the right, independently of the direction dashed. Whether this is intentional or a bug in the in-game physics is unknown. The same happens if they are hit by Dreamshield.
 * The explosive core cannot be damaged by the Spore Shroom or Defender's Crest Charms.

Notes 

References

Ума Ooma Ooma Ooma