okay, a lot of people do the same mistakes whenever staring TVD, TO or pretty soon Legacies fics, they get a bunch of names wrong

the most common? Klaus.

now, lets get everything on the table; Niklaus is his birth name, his parents call him that, Mikael calls him Klaus as a torment because Klaus shortened his name so that he could feel as though he had control over something, and most likely wanted to escape how weak he felt whenever he answered to the name of Niklaus. It's kind of like developing an alter ego, Klaus is his version of Batman I guess. (this is my own evaluation by the way, not off some tvd wikia or anything)

As for Nik, the only people that call him so are Rebekah and Kol, respectively.

(proof that it's happened more than one time and more than one time period)

Around season three of The Originals Aurora and Lucien drop by and some times use the nickname, but other than them, it's almost always Rebekah.

Elijah has the habit of calling everyone by their entire given name, so he always calls him Niklaus.

He refers to him as Klaus, as does his other siblings whenever he does something dastardly and Klaus-like.

I have a theory that for them, Klaus is the name for the monster that Niklaus unleashes whenever betrayed or angry or cruel.
Everyone else in the world only knows him as that monster so they know him as Klaus.

Actually, all the Mikaelsons have the tendency to refer to people by their full names, like calling Katherine 'Katerina' instead of her new name. Or referring to Cami as 'Camille' specifically Elijah in that case.

Another mistake is how you write Stefan. A lot of people write him in as 'Stephan' or 'Stefen' or 'Stephen' and just a multitude of incorrect spellings.
Listen, idk about you guys, but I personally double-check my facts whenever I write a fic, some times whilst in double-checking I take an hour on it and delay my writing process. It doesn't take too long to just google the characters and see their name written correctly.

And honestly, if I click on a fanfic and his name is incorrect I'll mostly leave immediately.

Another one is Katherine Pierce. Specifically, her birth name.

Now, it's written as Katerina Petrova.

There, that's it.

Again, it takes less than a minute to google the character and find the name written correctly, hell, even Wikipedia writes it correctly.

and finally, when it comes into your OC's name:

please take into considering the origin and time from where OC comes from

if they're from around when the Originals were human then her name isn't going to be Tiffany is it? Google doesn't less than five minutes.

For more information about appropriate names, please refer back to cresent00moon's book, I'm sure the link to her profile is in some other chapter around here.