Ooma

"Passive outer being protecting an aggressive, explosive core. Ghastly, eerie creature made out of a 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
It 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 enemy and dealing 2 masks of damage to the Knight and 50 damage to enemies. Ooma explosions will deal infinite damage to normal enemies but will deal 100 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 or Shade Soul projectile will continue 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 will be harmlessly destroyed along with the rest of the Ooma. Unfortunately, this method of killing does not allow Geo to be collected from the kill.

Weaversong, Dreamshield, Glowing Womb,  Grimmchild, and Defender's Crest charms will attack them, and the volatile core will still be 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 Sharp Shadow will cause its explosive core to always go to the right, independently of the direction the player dashes. Whether this is intentional or a bug in the in-game physics is unknown.

References

Ума Ooma Ooma