In my years of drifting through the Overlord, RosarioVampire, DxD and various crossover sections I have discovered one thing.
There are four kinds of OC's. No exceptions.
One - "The Background OC"
These would be considered one of the few acceptable forms of original characters that I can approve of. They can be exactly what it sounds like on the tin (barkeeps with no names, random civilians) or those that have a more prominent not-so-background role, but don't even step into the spotlight (characters that only appear once every blue moon, thugs, mooks, villains who get killed off in a laughable way after build up, or suit a weaker role).
Even with the name 'the background OC' these types can actually become really beloved. Ever played Fire Emblem: Three Houses?
"Greetings, Professor! Nothing to report!"
Two - "The Self Insert/Power Fantasy OC"
Semi-respectable, but only if the story is upfront about it. None of these "friend of Ainz's who just happens to be there" or "hey lets replace Issei so I can jerk it to all the DxD babes but I'm totally not OP and it's a legit story" crap. It's a guilty pleasure. You want to enjoy a power fantasy, replace Ainz or just entertain yourself. Be honest about it, accept that it IS garbage (fast-food style) and enjoy it. I hate these with a passion because they've flooded the Overlord section, but I accept why they exist.
I hate them, but they get a pass. Like McDonalds.
Just keep in mind, Overlord itself is one big power fantasy for the reader with a dash of comedy and dark tastes. Expect to be called out for your shit if you try to hide a story for what it is when you could just use the original cast. Many are here after exhausting the source material and craving more, so shoving these in their face may end poorly.
Three - "The Sudden Outta-Nowhere OC"
Ugh, I won't lie. I hate these. Now, what are these? Well, imagine you're reading a story about Ainz and the crew. Big adventure, then suddenly in, like, chapter seven they introduce a OC who joins them, just so happens to be able to match their power, romances them or just keeps popping up. In many stories, not even in just the Overlord section, these characters are typically introduced when the author is, unspoken, growing bored of their story. Every single time. They use these characters to keep themselves interested and keep pushing onwards, or get super defensive when people dislike the character.
You've probably came across a ton of stories like that in many sections. Think about it. Authors might make excuses like "it's important to the plot, or blahblahblah" but it's bullshit. You know it. I know it. The character is merely another form of power fantasy used because the author is bored and hopes nobody would notice. It's a hook for themselves.
Here's a tip: OC doesn't have to be a Mary Sue to be hated. Everything they do is judged by the reader.
Four - "The Cautious OC"
Alright, this is a big one. I call this 'Cautious' not because it's an OC that sneaks around, but because it's a OC that the author must write and use very delicately or wind up completely pissing off the reader. First, establish to the reader what kind of story you're trying to make. If it's a Ainz-focused fantasy like in the source material, then do not introduce a credible villain/hero out of nowhere. Make it clear in your summary exactly what you intend. If you want a OC to join the main character cast, as foe or ally, then do it cautiously, as you'll borderline tip into the other three sections if you're not careful.
Even introducing a OC as a love-interest for the main character must be carefully done. If it's clear your character is dipping into one of three other sections, hit the damn breaks. Kill it. Destroy it. Get rid of it.
Most importantly however, there is a big thing with this one. Do NOT think adding crossover characters without their powers/history/background are not considered OC's. If you write a RWBY story and there is a team with Izuku on it from MHA, but he just has his name/look and vauge nod to his powers - that's still a god damn OC and you know it. Don't you dare call that 'well I'm not using OC's guys, I'm using using repurposed characters from other anime'
Fuck you. We see through your shit. You're not being cautious at all.
STILL! If you want to avoid all these issues with OC's and more completely, just remember what I say next.
OC's are worthless and should die in a piss fire. If you can use canon characters, use them. That's what people are actually here for and nobody will care a INCH about your character when compared the fandom ones.
The "well, anime/manga characters were all OC's too originally" excuse is absolute nonsense and you know it.