I've been an original content artist since I started drawing. (I make fanart occasionally, but it's pretty rare.) I don't have a big audience by any stretch of the imagination, but I can give you some of the advice I've received, and what I've observed from OC artists who have made it big.
Style and Themes: Cultivate a consistent style which is able to evoke and convey mood. If your art makes people feel something, they'll keep coming back, even if they don't recognise the characters. (Hell, I follow a dude who only paints gorgeous landscapes with striking lighting and an... empty chair. There's always an empty wooden chair. His pieces evoke this warm, yet melancholic mood, which is why I love them.)
Consistency doesn't mean you can't evolve and experiment, but you want to always focus on the question "What is this piece giving to my audience?"
Skill: This one's a bummer, but it's really hard to grow a large following making OC art when you don't have strong technical skills. It doesn't matter as much for fanart, because the audience is more interested in the character than the skill it's drawn with.
I'm pretty sure this is why my reach is still so small, despite having been posting art online as a hobbyist for a decade-and-a-half. I just didn't have the skill required to attract a large following for the vast bulk of that time. I'm only just beginning to develop beyond that, and I've still got a long way to go.
Interest: If you want people to care about your characters beyond just thinking "oh, I like this art," you have to get them interested in those characters! Just focus on two or three characters to begin with, so you don't overwhelm your audience, and imbue every piece you make with as much of that character's personality as you can. Make illustrations which feature moments from their backstory. Make them emote. It's not enough to just draw portrait after portrait. Have them do stuff.
Ordinarily, people wouldn't have a compelling reason to care about my protagonist from Blue Star Rebellion, since he hasn't actually done anything yet. But, I've taken care to ensure that my opening pages have him oozing personality. As a result, I have an audience which is excitedly waiting to see what happens to him, because he's simply charmed 'em into caring. He's the only character in the comic so far, so it's all riding on him to be the hook to make people want to keep reading.
Make A Project: I didn't expect Blue Star Rebellion would grow an audience very quickly. (For the sake of clarity, I have about 100 readers spread across Tapas and WEBTOON, and I've only been posting for about three weeks. It's not overnight fame, but it's way better than I expected.) Blue Star's sci-fi genre is outside the one I typically work in, which is more typical, colourful fantasy. But, as it turns out, just starting a project will attract people, because it's giving them a reason to care about your characters.
If you're not ready to create a whole webcomic yet, start creating concept art for one. Make stand-alone illustrations within the setting, featuring the characters you'll one day write about. Draw parts of the story. Provide sneak-peeks and previews of what your comic will be about through these pieces.
But honestly, I'd say just... start making a webcomic. It doesn't have to be a masterpiece straight out of the gate. Heaven knows, I don't know what I'm doing. I've never made a comic before! Just start. You can always re-make things in the future, and the audience you have cultivated will probably be excited to see the story they're enjoying refreshed with updated art.
Good luck, and have fun creating!
