Sans x Frisk is pedophilia. I'm bringing this up since I see so many fankids of them. Plus I don't think Skeletons can have babies (I might be wrong)

Try to not have a million different powers without flaws/weaknesses. A tip I learned is that for every good trait, they need a negative trait to balance it. If your oc has 4 good traits than they need at least 4 bad traits in them to be balanced.

Don't self insert yourself into an oc, just write a self insert.

FOR THE LOVE OF NAGA, PALUTENA AND ANY OTHER GOD I'M FORGETTING COVER YOURSELF MY CHILD. That mary sue costume I joked about in my Arithony oneshot wasn't something made up. That's a good chunk of mary sue outfits. 

If Nekos are not a thing in the universe DON'T USE THEM. I SEE THIS SO MUCH IF I HAD A SHOT OF BLEACH EVERYTIME I SAW ONE I'D BE DEAD AND MY TEETH WOULD BE SO WHITE LEVI COULD EAT OFF OF THEM. (Don't do bleach kids. It's very unsafe). If your character has a cat eared headband, then I can accept that and won't consider mary sue

Don't have over 9,000 weapons at once on you. 

Don't go over the top in sadness. It's annoying and comes off as mary sue

Characters cannot bend to your will for your characters to be with. Eridan will always be asexual, Leon will always be gay, and Lukas, well he's another story entirely. 

Changing character names to name your oc after is a no no. Genderbend aus are different though and I'll allow it. 

This is just my thoughts, and you don't have to follow them if you don't want to. But just don't come crying to me after someone reviewed your oc fairly and called it a mary sue. I don't mean to hurt, I'm just being honest.