Oozers are zombie-like creatures that make their first appearance in "Simon & Marcy," during the post-Mushroom War. One appears in the Soupery, and two others appear in the Clambulance. Oozers are presumably attracted to loud noises, because when Simon knocks over the Clambulance, it makes a loud noise, attracting all the monsters to attack him and Marceline. Simon freezes them when he puts on his crown and creates a snow storm. They later appear in "James," where a large group of them are found in a pit in the Desert of Wonder (which is, apparently, the wreckage of the town Simon and Marcy lived in) where they attack Finn, Jake, Princess Bubblegum, and James. In the end, James sacrifices himself so the others can escape, is turned into one of them, and is shown leading the others out of the pit.
They are all various shades of green, brown, and yellow, and blue, and have various numbers of holes in their head for their eyes and mouths, out of which oozes a glowing green substance. The ooze that comes out of them is very corrosive as it's seen melting through the glass window of Princess Bubblegum's tripod lander in the episode "James." Princess Bubblegum also claimed that she detected large amounts of radiation in them. The ooze is likely the same substance seen in "The Vault" (when Shoko fell into the green ooze river), in "Mortal Folly" and "Mortal Recoil" (when Princess Bubblegum fell into the Lich's green ooze), and also in "Jake the Dog."
- Oozers are very similar to classic movie zombies as they are relatively slow moving, will relentlessly attempt to bite living things in order to turn them into monsters, can only make groaning noises, and groan loudly to alert other monsters to their prey's presence.
- Oozers seem to mainly rely on their sense of hearing. They are attracted to sounds and loud music (the "Clambulance" crashing and the "Happy Broth-day" card). They also appear to use sound to navigate (bumping into the shelves before Simon drops the Broth-day card, then reacting to the sound). However, they are not completely blind since Princess Bubblegum used the Tripod's flashing lights to help distract them in "James."
- Since "Simon & Marcy" is the earliest we know of the world post-Mushroom War, the Oozers are likely one of the earliest recorded (if not the very first) mutations caused by the Mushroom War.
- It is also likely that they were once part of the Lich's undead army.
- Oozers appear to be former humans mutated by radiation, since large amounts of them are encountered in former populated areas such as the city Simon and Marcy are in.
Appearances in other media
- An oozer appears as a non-playable character in Adventure Time Battle Party. He is a powerful monster who lives near the altar on the top lane. Upon being defeated, will grant a temporary increase in movement speed to the team who slayed him.