1. try not to put your character into situations you know that they can't win.for example, your human oc isn't realistically beating klaus in a hand to hand fight. it's just not going to happen.
even if your oc had the white oak stake, i still don't think they'd be able to beat klaus. though their odds might improve slightly because he may be more cautious.
does that mean your oc can't fight klaus and win?
no. that's not what that means. it means you need to be creative and figure out a reason why your oc wins.
for example, in my fanfic my oc fights klaus and wins. now she's a vampire and a psychic and she was trained by klaus to fight so she does have better odds, but she doesn't win because she's a better fighter. she's not. she wins because of the situation of the fight. i don't want to go into too much detail over what happened in case anyone wants to read but to sum it up, her and klaus were best friends and he feels guilty over what had happened to her and what stopped them from being friends. so he can't win against her because he has no desire to actually hurt her.
2. find a way to get them out of a situation if they're in one they can't win.
for example, elena using the cure against katherine gave her in advantage. it allowed her to win. if it wasn't for the cure, she probably wouldn't have won.
i could be wrong on exactly how this scene played out but in the originals when klaus is knocked out and cami is trying to defend him, obviously she would not have one. but then hayley shows up. marcel too and i think elijah. and klaus wakes up. well, now the odds are that cami will be fine because she has 2 originals, a hybrid, and a vampire with her against mikael.
if you're writing accurate to the show, intense situations are probably something you want. you can put your character in one and still make it so that they survive without it being plot armour.
3. give reasons as to why your oc is protected.
for example, i know a complaint i see people have about the show is that the mikaelsons never retaliated on the main characters for trying to kill them and for killing kol and finn.
the main characters had tons of plot armour when the originals showed up and i know a lot of people felt that shouldn't have happened. it wasn't really justified.
however, some of it was.
for example, when klaus needed elena to make hybrids, he couldn't kill elena because then he couldn't make hybrids. so elena was safe and it made sense.
i think the thing about plot armour is that it's more "this character can't be hurt because they're needed for the plot" rather than things being written in to allow the character to continue in the story.
so i think if you can gain reasons as to why your oc or other characters is protected, that'll make it seem more accurate to all of the characters.
as i mentioned, elena can't be killed by klaus in season 3 because he needs her blood. though it's not very clear if that's what he wanted, one could argue that was why she was protected in season 4 because he could've thought she'd take the cure and become human again. jeremey couldn't be killed without the character having to undergo that curse.

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the vampire diaries fanfic writing tips
Fanfictionhere you will find some writing tips for writing fan fiction for the vampire diaries and the originals! includes: advice on certain tropes, tips on writing certain characters, writing characters as love interests, breaking from canon, general quest...