Original Characters (or OC's) are characters that are not already-existing in any possible way and are 100% exclusive to the creator or animator. It is the key part of making an animation memes, although it's a universal term referring to everything related to characters that are not already-existing.
They are often mistaken as Fursonas, even though there are hard and distinct differences between an OC and a fursona in terminologies.
They are the key feature in animation and art memes.