M-O, short for Microbe Obliterator, is a diminuitive robot who lives and works on the interstellar cruise ship, Axiom. His duty is to clean, and nothing more. He takes his job very seriously, and despises dirt to the point that he will remove it the instant he sees it. As for getting dirty himself, he is known to scream if such an incident occurs, as proven when WALL-E ran a filthy tread across his face. Ordinarily M-O will keep to the light track laid out for him, but can and will break this rule if there's cleaning to be done.

Initially brought in to clean the EVE probes sent out in search of signs of life on other worlds, M-O realised he had his work cut out for him when he encountered WALL-E, covered in several hundred years' worth of dirt. After WALL-E left, in pursuit of his beloved EVE, M-O followed, driven by a need to clean every scrap of 'foreign contaminant' from the little robot. Only when WALL-E was badly injured and slowly dying in the garbage bay did he keep still long enough for M-O to do his job. Immediately afterwards, WALL-E offered his hand to M-O in friendship, which M-O shook in response, after quickly cleaning it a little first. M-O was last seen back on Earth shunting a group of malfunctioning robots away as they watched a rebuilt WALL-E and EVE settling into a long and happy relationship.

He can also be seen in the film's credit sequence cleaning away the trails of dirt a WALL-E graphic leaves as it rolls around the screen.