I have several OCs (not counting original), four of them are from fandoms (in order of creation):
Goldicat the Cat (Cat Animatronic from Five Nights at Freddy's with a fun-loving, slightly annoying and perky persona). Goldy or GC for short.
"A golden and brown cat with soft fur and milky bluish grey eyes, sporting a black bow."
Rinji Shukei (Soul Reaper Captain {The Lotus Squad} from Bleach with a fiery temper and sassy, tomboyish persona). Rin or Rinny for short.
"A short, dark brown-haired young woman whose hair reaches her lower back in a large ponytail and has green-blue eyes like sea-foam."
Asia Abernathy-Lovegood (Hogwarts Student from Harry Potter adopted by Luna Lovegood with a smart and cool-headed persona). Aggy for short.
"A black-haired, Hime-cut young Japanese girl with harsh yellow eyes."
Alexandra Jackson (Friend of Charlie from Five Nights at Freddy's book series with a artistic and adventurous persona). Alex or Alexis for short.
"A dark red-haired, black-highlighted young woman with her hair in a straight Hime-cut with bright grey eyes."
Creating an OC is like creating anything: you have to start somewhere. Before you start, ask yourself: what fandom do you like best? Star Trek, Star Wars, Harry Potter, etc. What part did you like best? What group did you like best?
Ask these questions, placing your OC inside each. Once you've decided, decide on hairstyles, or colors, etc. What about fashion? Try to keep him or her true to the fandom, like the creator him/herself made it.
Have fun creating!
With Goldicat, I created a cat character because FNAF doesn't have any cat animatronics, I created Rinji because I loved Bleach, I created Asia for a story, and I created Alexis through looking up hairstyles. Different ways but still! Have fun.