If a story features a supernatural OC and the main guy is normal, that'd definitely catch my interest. It's hard, however, to pull off an OC that's mysterious and powerful and dangerous all at the same time compared to when you're writing about the guy, haha.
Just an idea at the top of my head for you to use/work with if you'd like: the main character could start off antagonistic, in the sense that she's engaging in anti-human behaviour (killing people, hurting them, etc.) because it's in her nature. But a big, unexpected turn of events occurs when she's on the hunt again, and it could be the main guy that's involved.
Maybe try reading a few supernatural fics here or some actual published novels for inspiration. Anyway, all the best with your story and writing! I'd like to see it if it becomes published.

I've not read any stories (maybe it's just me) with OC as the supernatural one. Hmppm You could give it a try? XD

Okay... The OC is little bit crazy and loves adventure and danger... One time she heard something about Vampires really do exist and she start her journey to find one... She searches through old abondened houses, castles, caves and burial places(?)... And finds one.. when he was trying to suicide by burning under the sun... That one being is gloomy Suga... He is... maybe 400+ years old.. and has a brother... And that's faced Sehun.. he hates humans and wants nothing to do with them... But wants his brother to be happy... By meeting her Suga started to change.. little by little.... And that was enough reason to Sehun not to kill her.... As time passes, her love..or need to find adventure increases.... So she goes to find werewolf.. even though she said she wants to go alone, Suga and Sehun couldn't let her go alone.. since they know how she can be crazy at times... LOL.. all the time...... And she finds lone werewolf named Ravi... He finds peace by being alone but he's not all gloomy and angsty.. he just loves nature... He's been alone for 5 years and was starting to want accompany... And our 3 happened to be that..... Then suddenly she wants to live in a big castle with her friends... Then Suga, Sehun and Ravi takes her to haunted castle which no one cares to come close... But for her, it was new adventure... They starts to redecorate whole place... When night comes, she meets with the said ghost... Jackson the naughty elf.... (As you can see, I love reverse harem :P ).... Her friends are good people/beings... But sometimes they forget she's human.. so she decides to find human friend who can accept her other supernatural friends.... And that happened to be Jooheon, Wooshin and Yuta... The vampire hunter, werewolf hunter and magician..... LOL look at her luck.... She gets ambitious, so starts business which deals with supernatural concerns... Through her life, she found many friends with different abilities.. like doctor, clown, musician, farmer, detective..... (Good luck XD if you really do write it, please send me the link :P )