The OC is the half sibling of Anthony Larusso and Samantha Larusso, since her mom is Amanda Larusso she was born from a one night stand and she doesn't know who her dad is. Sam and Daniel don't like her for that factor, so they are mean to her and Amanda is the only one on her side.
She'll be in a love triangle with Miguel and Hawk in season 1 she's make her choice in the same season and I'll figure out the rest. She joins cobra kai for Miguel from the start.