This part is fairly simple, first, let's start with 3 basic ones: Kind, rude, & Spaced-Out (aka: dumb)

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Now, let's think of 3 basic scenarios:
First one: Someone walks up to your oc, they then start a friendly conversation. How does your character react?
Do they act rude and either scoff or walk away?
Are they kind and engage?
Or are they too spaced out to realize someone is talking to them?

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Depending on your answer, it will help decide the oc, for example, let's say you chose the rude one, & they scoffed and or walked away, you have the design of a rude character, let's develop it from here.

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Why is your character that way? Let's continue our example of the rude character:

Are they too self-important that they don't have time for people they consider to be below them?
Have they come from a broken family, and perhaps they never experienced kindness so don't know how to return it?
Do they enjoy hurting others?
Or are they just so hot headed the littlest things will set them off?

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Once you've answered those questions, let's really develop their personality. For example, let's say they're too self-important (aka entitled).
Why are they like that?

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Were they spoiled and became bratty?
Do they expect everyone to treat them as some god/goddess/other?
Or did they become really good at something (example drawing) and now think of them self as some celebrity?

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Of course, there are other options, but these are the main ones that lead to brattiness.

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Now, maybe they weren't spoiled, perhaps they're just a tsundere.
Tsunderes are fine, but here's a question: Why are they a tsundere?
Did something happen for them to become one, or are they just like that?

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Now, you should hopefully have a personality idea, we can move onto my favourite part, designing~

Designing ocs are lots of fun!
There are always a few things to think about when designing.
Like: Where do they live? Is it hot? Cold? Dry? Wet?
If they live in a cold area, perhaps they'd like to dress warmer.
How's they're personality?
Perhaps, if they're more entitled, they wear lots of fancy expensive clothes.
Or, if they're more on the uwu side of things, they dress cuter and more innocently.
That being said,
you could always make them the opposite, a mean entitled rich person who wears more cheap clothing.
Your character, your design. :)

This is a bit more tricky, so let me simplify it,
Events that will affect your character.

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Let's take that entitled character we were using as an example earlier, perhaps while shopping she sees a person barley scraping by, shopping for clothes to wear.
Maybe that person doesn't have enough money.
This is where our character comes in.

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Does she pay for it for them or laugh?
This can go 2 ways: Either, she realizes all the money she's about to spend without blinking after seeing someone who can't afford a simple tank-top, and puts her items back and offers to pay for their clothes.

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Does she laugh? Is she so entitled she finds it humorous?

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Let's say she laughs, maybe she then sees the person as a child walks up to them with a jacket, perhaps she sees the person put their item back to pay for the child, let's say it's also quite cold, so that person goes without a jacket while their child stays warm.

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Perhaps, she has a change of heart, maybe she remembers her parents buying her clothes for her like that parent did for her own kid, and decides to buy the parent the jacket.
Or, maybe she just laughs it off, but on the way home she thinks more and more, returning to her house to throw the clothes to the side she'll never wear, and realizes how nasty she is.

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Of course, she could always not care, but that means no character development which is bad.
There always has to be some form of character development for a good stable oc.

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Now, avoiding making an OP Oc/Mary sue. Mary Sues have practically zero flaws, are great at everything, and usually never lose, this is what I like to call a: Dead Character.
They don't have flaws which is what gives your character development, which is what I was referring to up above with the whole nasty thing.
It's fine to want strong ocs, but give them some flaws, maybe your character is a strong demon, but perhaps they're weak to let's say, sunlight, it keeps them lurking at night which is interesting.
This way, you can have a strong oc and avoid a mary sue.

This step is to avoid what I call: Pity Characters/Rpers.
These are the types of ocs that always have, depression, self harm, bullied, bad past, and or hated.

These types of ocs usually belong to attention seekers or people who don't know how to make a good oc, yet.

So to avoid this, maybe overtime have your pity oc become less.. pathetic. Maybe they stand up to their bullies and become happier.

Or maybe they find someone they love and forget about their awful past and become happy.

Also, DEPRESSION ISN'T A PERSONALITY TRAIT!