He can take photos of the main oc everytime he sees her, follow her, collect some of her things in the trash, have her phone number by using her friends (just popped in my mind), disguising himself as a girl to be close to the main oc (that would really be creepy)
That's just a request if you are doing a bad boy character please add some possessiveness and that hard to get attitude :) anyaways i hope that would help you fighting author-nim

Beat up any person (male ) who tries to come closer to her or flirt with her but being in his limits and not making it awkward or even any guy she is comfortable with.
You know possessive freak stalker .....
Let me know if you like or understood my pov
I will explain it better

Maybe the character could follow her home, watch from a far. Maybe he lives across the street, and watches her from across the building. Whatever u do w/ ur character, this sounds like it's going to be a good fic. :)

Act all nice to her face but follows her home, tracks who she talks to even manages to gets under her bed without her knowing. watches her in the shower. Haven't you ever watched other stalker movies. here i'll recommend some; Orphan (R), Fear 1996 (R), Obsessed (PG-13), and then there's Stalker the TV series. Don't judge me I was doing research myself -.-

random presents for her at her doorstep and (like many said) notes. and they get more and more intimate, like, at first toys and cute books and suddenly there's a blindfold or toys, yk. maybe that the notes start out cute and well written but when she gets scared they get more and more messy and persistent and maybe even written with blood. (There's this one rly good fic called 'written in blood' at least I believe it was IN and not WITH or so) so, going on... mhh... maybe phone calls where, when she answers, no one talks. and maybe the main chara could at some point always see someone close and watching or following her in distance... such stuff. and maybe her stuff vanishes. like, her underwear or the stuff she last wore, vanish one by one after she upsets the stalker and gets the bloody notes. or maybe one day she comes home and finds her whole room with the walls plastered full with pictures of her, and those with other people on it have their faces scratched out. mhhh. oh, and don't ask why I think of such stuff, I always have... sicko ideas for fics xD

like umm first, start from letters in the locker to letters and flowers and stuff on her desk, and then after a bit, when she gets home from school, she finds gifts and a teddy bear, with a note saying, "My love, I've found you!" then end with a mysterious name initial like ex. "My love, I've found you! - PCY" and after that she's really freaked out and goes over to her friend's house and like after a day or so she finds another gift on the friend's house' porsche with a note that says ex. "My love, did you really think I wouldn't be able to find you? - PCY" and continue from there, with your amazen mind

Um have him appear to her randomly. Or she sees him everywhere, like every time something happens he's there

You should read the girl on the train and you will know what stalking really means.