Oddly colored hair (naturally or dyed) ie: blue, pink, streaked hair, "bottle red" (Think Rihanna)
Oddly colored eyes (and colored contacts) ie: red, yellow, pink, COLOR-CHANGING EYES
Unnaturally beautiful characters, ie: perfect proportions and complexions, etc.
Being an animagus or a metamorphous (meta is very, very rare and ani is hard to achieve and you have to be registered)
Being a half-breed creature, ie: half-veela, half-vampire, half-werewolf (or a werewolf, for that matter)
Having your character self-harm, ie: cutting, popping pills, etc. (This is a very over-done thing in oc's, and it's frankly not something healthy)
Having them dress in clothing outside the time-period (do your research)*
Having them be goth/emo/scene (seriously over-done)
Giving them tattoo's and piercing (they go to a boarding school in Scotland, seriously guys)
As just a general statement; "Scene", and "Emo" fashions didn't start up until the mid-to-late 2000's. Mind your time periods when creating characters.
*Wizarding clothing is more of a Victorian-influenced style (according to the movies and books), Muggle clothing would be early 1990's to late 1990's
Most clichéd romances for your oc to experience: Harry Potter, Draco Malfoy, Severus Snape, Remus Lupin, Sirius Black, Fred and/or George Weasley, Ron Weasley, Neville Longbottom, Luna Lovegood, Ginny Weasley and Hermione Granger (basically, main characters. I'm not saying all of these romance stories are horrible. I've seen some wonderful ocxcanon pairings)
THE WORD OF JK ROWLING IS PRETTY FINAL; we know who marries who and who dies and such for the most part. Please stick to the storyline.
More clichés: having your oc sleep with a canon character, be raped by a canon character, dating/loving a teacher (anything past a crush), having a canon character's baby, marrying a canon character, etc.
You can always make up your own love interest for the your oc, you can mold them into whatever you want and you're free to create your own storyline without having to worry about sticking exactly to the book.
Family and Friends:
Killing off your oc's parents is so overdone it's not even funny (~90% of all oc's I've seen are orphans)
Having Voldemort kill the parents is even more of a cliché
Having them grow up with horrible guardians or having them raise themselves is another big cliché
Having them be sexually/mentally/physically abused in their childhood is very cliché
IF THEY ARE THE TWIN OR THE SIBLING OF ANY CANON CHARACTER, THROW THE IDEA IN THE TRASH AND START AGAIN (seriously)
Having them be closely related to canon character's is commonly overdone as well
If you're making them pureblood, do a little family tree research. I looked alot at family trees and such, and it helped me understand relations much better
If they are the child of any of the following, throw the character out and start from scratch: Severus Snape, Sirius Black, Remus Lupin, James Potter, Dumbledore, Voldemort and Hagrid
If they are friends with Draco Malfoy's crew then they are probably not going to hang out with the Golden Trio, and vice versa. It's just not very logical.
They are not on the Gryffindor Quidditch team while Harry Potter is attending school
No girls are on the Slytherin Quidditch team while Harry Potter is at school
YOU CANNOT USE ELECTRONIC DEVICES IN HOGWARTS
Uniform violations are getting to be ridiculous (check out my uniform guides for info, posted below)
No wands with Phoenix feather cores
Giving them a wolf/dolphin patronus is very overdone (no mythical creatures as well, like dragons and unicorns)
They cannot have a pet at school that is not an owl, a cat, a toad or a rat
Transfer students would be very unlikely (American and Japanese transfer students are so overdone)
Gryffindor:possible the most clichéd house, "Brave, loyal, sometimes stupidly courageous, etc." Colors are Red/Gold, Boys cannot be in Girl's dorms.
Typically the "good" house, where all the heroes and heroines are. They might tragically die during the Battle of Hogwarts and then are later brought back to life.
Slytherin:Another clichéd house, "ambitious, cunning, etc.", Silver/Green colors, very blood-purist.
Typically, this is the "bad" house, full of the party girls/boys and the "rough" crowd. This is a split when it comes to the Battle of Hogwarts though. Either they're Death Eaters or they fight with the good guys and almost die for a canon-character (usually Harry or Draco). Usually labeled as "misunderstood and emo/goth".
Ravenclaw:not as clichéd but it has it's few sues. Usually portrayed as the "nerd" house because it's full of intellectuals and people who love to learn. Colors are Blue and Bronze in the book and Blue and Silver in the movies.
Hufflepuff:The least-clichéd house. Very underrated house full of loyal, hardworking students. Colors are Yellow and Black.
There are no other houses at Hogwarts besides these four, unless you count the fictional Mary-Sue house of SparklyPoo (: