Okay so, I might not be the right person for this, cause I also am not all that great at drawing in the Spooky Month artstyle, but I did some character design at school so I might be able to give a few tips! Please do take everything I say with a grain of salt tho!
Spooky Month's strong suit is definitely character design. It's SO expressive and well done it's genuinely unreal. You might want to know exactly what your character is gonna be doing, or how their personality is gonna be like, because character design in Spooky Month cares a lot about things like these. For example: Radford works at a cinema, so of course he's wearing 3D glasses, but also his HAIR IS LITERALLY SHAPED LIKE A POPCORN.
So maybe decide on a job for your character, and try to start from there, get crazy with shapes and have fun! Spooky Month characters have designs that are both extremely simple to draw, since they use mostly basic shapes, but also are extremely thought out and meaningful. Another example of great character design is Pump
Now, I have no idea if you already had an OC in mind, but make sure to choose the right shapes to represent it. Something I see around in the fandom is people making these OCs that are like, serial killers and dangerous people and stuff, but then give them the "Lila"-like oval head. And honestly, nothing wrong with that, that's a choice you can use! But still, shapes allow us to understand a lot about how a character is just by looking at them. So you might want to experiment around a bit!
Ovals in Spooky Month are the "good" shape, let's just say. Most character with an oval head are sweet, helpful, kind-natured! It's often paired with oval eyes, so it's mostly a shape that's used for not villainous characters.
Circles are a bit more complicated, because Spooky Month subvertes the Circle Characters. While yes, they're also used to draw children, such as Skid and Pump, so they may come off as unthreatening at first, most main villains, such as Eyes, Bob and Dexter Doll (which is meant to represent the likes of a child, so that's a nice contrast), are mostly circle-shaped.
There's a very wide range of Square characters in Spooky Month. Square characters are usually bulky, big and strong. They often come off as threatening (such as Moloch), but there are so many other fun things you can do with them. Like, take Frank. EVERYTHING in his design should alarm us, him being square-shaped, the black eyes, the wide smile. Yet, he has a shape of the eyes that's very relaxed and chill, so we end up trusting him. As for Dexter, he's a mix of circles and squares, so we can't really understand his intentions right away, because he's shaped in the most confusing way possible. He's just made to be unsettling and leaving us to wonder if he's a bad guy or just an oddball.
You can do SO MANY fun things by mixing up shapes of faces and eyes it's INSANE. Also, you can mix up other characters' features to create a new one, if you're planning to do a fankid or stuff like that. Look at Ross, he's literally a mixture of all his parents' features!
My main tips for drawing in the Spooky Month artstyle are mostly
1) Play around with shapes. Be as cartoony as possible.
2) Try to be consistent with proportions, because, based on personal experience, if you draw the pupils of the eyes slightly off it changes the whole character's expression drastically
3) don't worry too much about details, Spooky Month has a very simple artstyle. You don't have to draw a perfectly anatomically correct hand, just whip up some cartoony three-to-four fingers and you're good to go
4) try to redraw some pre-existing screenshots from the serie to get familiarity with the way Pelo draws expressions. It helps a bunch.
So yeah, that's all! Good luck with your oc :)