Like OwO, it’s supposed to be a letter emoji to describe the face of a smiling cat, but for OmO it means the cat is frowning—conveying emotions of sadness, concern, disappointment, or annoyance.
Furry: You wanna go out for dinner?
Friend of furry: No, sry, I’m in the hospital recovering from a car crash.
Furry: OmO………O,M,O !!!!!!!!!!!!! ARE YOU OKAY, MAN~!!!!