Things to avoid:Canon Sues: These are extremely annoying types of Mary Sues. They are based entirely on a Canon character. If the canon character has blonde hair, so do they. If the Canon was stabbed, so were they.
Long names: "My OC's name is Marie Angel Raven Diamond Sparkles Rose Emaralde Trinity, but she goes by Trinity!" Really, people...
Horrible Past: (These is ok for Creepypasta OCs) "Trinity's after raped her when she was six, and she ran away and was involved in a tragic car accident, but survived because the is immortal!" Tragic pasts are one of the biggest Mary Sue traits.
Bi Colored Eyes: Unless there is a reason for this, such as a half blooded character or genetic mutation, bi colored eyes are a dead giveaway Mary Sue trait.
Perfect relationships with canon characters: This goes for romantic relationships and friendships. If a character gets along perfectly with a Canon character, that often means that the character is a Mary Sue. I'll use my LoZ OC, Bree as an example. She gets along fine with Link, but sometimes grosses him out by eating fairies. They are not in any romantic relationships.
Rare species: Nekos, Angels, Alicorns, etc.
Mutant blood. [Homestuck]: This is one of my biggest pet peeves. Why do you need to have a blood color as rare as candy red?! Or even worse, something rarer? It just screams "Mary Sue."
Immortality: Everyone dies. Even Cana, my chicken character who is a goddess dies at one point! There is no way you can live forever [Unless most characters in the fandom are immortAl]
Too many talents: "My Oc can sing, dance, paint, fight, do gymnastics, play 5 sports, and can play 29 instruments!" No matter what, no body can do all of these things. It's just too many talents.
Magic: It reAlly pisses me off when OCs are overpowered. Ocs should not have more than 3 powers .depending on the fandom.
Enchanted objects: These are fine for some fandoms, such as Legend of Zelda, because other characters have them, but a My Little Pony OC with a magical flute!? Few characters in that fandom have enchanted objects.
Common weapons [Mostly creepypasta]: If all OCs were canon, imagine how many would wield knives. Really!? Use a less popular weapon, will you?
Everyone loves them. Look, everyone has enemies. No good OC will have the most fierce character swooning over them, or the most sinister villain calling them "-Chan".
Insanely beautiful.: "Beautiful blonde locks, amythest orbs, rosy cheeks" This is the description of a Mary Sue, unless this is your style of writing.