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1.
A character in a major franchise created by a fan of something, and self inserted into that franchise usually via art and fanfics.

Usually due to the fact that the designs end up being so close to the source media, they end up looking almost like the original, and a ton of OCs are just traces/recolors of offical artwork for the source franchise.

Many of them also happen to be Mary Sues with self insert fanfics and copy/paste backstories.

The creators of them are also known for being very egotistical about them, going as far as putting copyrights on them, telling others not to steal their (possibly stolen anyway) character, and even abusing the report systems at various sites just to be sure nobody will steal their OC.

They exist for almost any franchise, they can exist in everything from Sonic to Pokemon to Naruto to even South Park!

With some franchises, such as Sonic or MLP, there are even OC generators that people will use to generate OCs and put them up on sites like DeviantArt as "art".
Blonic is my original character, DO NOT STEAL!
by Mjfan13298 May 17, 2012
 
2.
An excuse to make horrible fan art and parade it around like it's the best in the entire world.

Others will go to great lengths to outdo the person that claimed theirs. It eventually turns into a Vicious cycle with people coming from different forums/places to outdo everyone else. The ending is decided by who steps away and accepts defeat first. Which almost never happens.
Brad: Hey! Look at my SUPER AWESOME 1337PWNAGE MLP Original character! I call her Asmorphilus and she's--

Ray: Woah woah wait hold up there, my original character is WAAAY better than yours! Her name is---

*Both proceed to fight and argue like kids over which is better*
by TheTankMan January 12, 2014
 
3.
An anime character of yourself, created by yourself,
"Man I'm going to make my own anime character of my self!" "You just made an Original Character"
by KatieRoolz August 27, 2008