Here's something I've been thinking about and struggling with for a long time: where do you draw the line when it comes to the difference between an OC and an NPC? Do you draw a line at all?
For example, let's say you have a story/universe that takes place at a school. Therefore, you have to have teachers! But you're not really interested in the teachers' stories, so they're pretty much setpieces. They have a design and maybe a name, but you don't really ever think about them, draw them, write about them, etc. When they appear in the story, it's pretty much to be in the background to remind you that the setting is a school. Some teachers might have development and character, yes, but this one doesn't get that much! The main character doesn't even have classes with them! But maybe there's one scene where their friend is in said teacher's class, warranting the creation of the teacher's design. So... is the teacher an OC of yours now?
As another example, Nolan here is established to have a family of decent size as well as some college buddies. However, outside of his father, the rest of his family and his friends have 0 relevance to the story, so I haven't bothered giving them names, designs, or anything else. But they exist and the story acknowledges that they had impact on his life. So, should I consider them OCs? If not, what if I have Nolan tell someone one of his wacky med school tales and make up a name on the spot for one of his friends? Since that character has a name and some action(s) they performed once, does that make the friend an OC now? What if I have Nolan own a photograph of his family and show it in a visual medium and have him mention the name of a sibling? Should I count that sibling as an OC since they have a design and name?
And don't get me started on making a video game! Would you consider shopkeepers your OCs? What if they all have unique designs, but still no names or notable personalities? What about the innkeeper? And what about enemies? Would you consider each unique enemy design as an OC? Think about your favorite video game, and then think of all of the NPCs in it. If you made that game entirely by yourself, would you consider every design in it your OC?
The list goes on and on! If a character flirts with a barista, is the barista an OC now even if said barista never appears again in the story? Is your character's ex an OC even if their only establishing trait is "they dated this character long before the story began?" Is that bully from high school your character offhandedly mentioned shaped their current personality an OC? Are characters/monsters of the week your OCs?
I made an OC list recently, but I cut out a lot of people because I felt like they weren't really OCs, and instead were just random people who existed to fill out the universe, like random designs to work at a store or enemies of the week. There are more characters I feel like I shouldn't count, too, and others I worry have too much influence on the story/backstories of various OCs to Not count as an OC even if I have 0 interest in them and don't plan to even give them much of a personality, much less anything else.
I don't have too much input here, because again this is something I've struggled with for a long time! It makes all of my OC lists a mess and I always feel like they're simultaneously incomplete because I didn't count Literally Every Family Member and Backstory Character Ever (even if they're dead) and too bloated because I've counted characters I never plan on doing anything with. I'm hoping by seeing others' opinions it might give me a better idea on how to define OCs for myself.
So! What about you? Where do you draw the line? How much character does a character need to have for you to put them on your OC list? Are all "NPCs" OCs to you, no matter how unimportant? Does a character need to have a personality before you consider them your OC, or do they just need a name and/or design? Are you just as conflicted as I am?
Remember, this topic is super subjective! Your answers can only really be about what line you've personally drawn, since as far as I know there's no big ol' enforceable list of OC laws. So, don't be rude to anyone if their opinion on what an OC is conflicts with yours! Not that I'd imagine this is a particularly heated subject, but it's worth mentioning!