Judges and your general audience alike are typically looking for three major factors when they read your audition: Interesting Characters, A Unique Plot, and Clarity of Storytelling. If your audition does not excel in at least one of these areas you will almost certainly be looked over when it comes time for judging no matter how slick and professional your art is.
I firmly believe that there is no such thing as a bad character. Any character, if presented well, can be interesting--be they Human, alien, psychotic, or cliché.
The trick you need to employ is treating the character in a way that supports them. Characters should not be the sum of their gimmicks. They should have depth, personality, back story, and motivation. This last one is especially relevant to auditioning for an OCT; it is your character's motivation that will get them onboard with the tourney concept and it is what will keep them moving forward in the story.
But Judges want to see variety, not the same character concept done over and over and over. We get sick of charming sociopathic murderers and nice characters who have an evil, psychotic second personality after we've read ten or twenty auditions starring such. If your character fits into a common theme you are going to have to work much harder to sell that character than if they are unique.
If you've only created this character in the first place because you saw some one else become extremely successful by championing a character of the same theme, then you are doing it wrong.
There is a tendency--when writing an audition--to go with the most straightforward plot, but Occam's razor is not useful here; the fastest route between two points may be a straight line, but that's also the route everyone else is taking. Your character is doing their thing when suddenly a representative of the tournament takes notice of them and invites, bribes, or threatens them into joining the competition. It's pretty easy to fall into this category, because it's so obvious. While there's nothing inherently wrong with it, it also does not give your audition any kind of advantage.
If your story doesn't bring anything new to the table, then it's up to your character at this point: if your character is phenomenal, unique, or supremely relatable using the same formula as everyone else may work for you. If your character is common or cliché you will be overlooked.
At the end of the day, however, judges and readers are always looking for something we haven't seen a hundred times already.
Auditions should also be about showing the judges what you can do in the context of the tournament you're trying to enter. If you're entering a fighting tournament it is fairly important to give an example of your ability to draw action, or at least some sort of demo of what your character, specifically, is bringing to the table. Don't, however, shoehorn fighting into your audition just for the sake of having an example of a fight. You're still telling a story, regardless of the fact that you're telling a story with specific parameters.
Clarity of Storytelling
Clarity is probably the most important thing you can get right, though if it's done well it probably will not be noticed at all. Clear storytelling and strong narrative are the unsung heroes of comics, while not understanding what is going on in your audition will instantly turn your readers off and make it impossible to consider you for a roster position.
There are a number of ways in which your audition can benefit from clarity or suffer from a lack of it, but the most common problems boil down to lack of context.
You can't assume that your reader knows anything about your character, the universe they live in, or the universe they'll be entering for the duration of the tournament. Don't tell inside-jokes that only your friends will get, but likewise, don't tell inside-jokes that only people who have read the NPC reference sheets, or hung out in chat rooms with you, or have read your prior work, will get. Assume that everyone reading your audition is coming in with a blank slate.
In the same vein, there is a tendency to forget that people don't necessarily know what's going on visually. Readers are automatically hardwired to think in terms of either real life or familiar genre tropes, and if the contents of your audition don't fit inside either of those then not giving them a context will be confusing and distracting. Essentially, you need to present weird things alongside normal things so that people know what they are.
Sense of scale is the first thing to go. If aliens or robots show up in a comic and are never seen on panel with anything familiar, we have no idea how big or small they are. Is that robot three feet tall or seven foot two? Use familiar things as guides. Your character, a car, couch, lamp, or doorway are all frames of reference we understand. This problem isn't limited to the fantastical, either; anything that we're not immediately familiar with should have something we are familiar with nearby to gauge it by. Whether you're showing us explosions, "the fish was this big", your character's bedroom, or a frozen wasteland, context is king.
Other stuff to consider:
If you're doing comics then it is crucial that you learn how to do speech bubbles. There is a tendency to try to make speech bubbles the focal point of a comic, when you would actually benefit from aiming to blend them into the background. Good speech text is subtle and obvious at the same time; in an ideal world you want your reader to read and absorb what your characters are saying and then forget that the speech bubbles are even there. The more work you make people do to read the dialogue the less they are paying attention to the contents of that dialogue.
If you hand letter than you better be using your best handwriting; if you use fonts than the less gaudy the better. Fonts need to be legible, if you're not sure, have someone who hasn't read your script look at your comic and let you know. Avoid cursive style fonts or anything with a stroke. Use tails whenever possible. While assigning different fonts or color coding the speech by character can be helpful, you cannot assume that your reader will automatically know which color/font belongs to which person, or can even tell the difference between the colors/fonts you've chosen.
Google "speech bubble tutorial".
I said this once before but it bares repeating: Being a good artist is not enough. Not every judge is as finicky about good writing, and sometimes a poorly written but beautiful comic will make it into a roster. This is a competition, though, and once you're in the roster your pretty art is going to be compared directly against someone else's pretty writing. You shouldn't be surprised when you find out which way the voting goes.
Conversely, if you draw a niche artistic style, such as animals, robots, or adorable kids, keep in mind that the majority of your opponents won't fit into your distinct, preferred category. If all you draw are lions, how do the judges know that if they let you into the tournament you'll even be able to draw your opponents, who will probably not be lions? Be prepared to draw anything.
Average screen resolution for computers is 1024x768. Not long ago it was 900x600, so in a few years it'll probably be some ridiculously huge, wide screen resolution, but right now the average screen resolution for computers is 1024x768 and if the body of your entry is wider than about 950 pixels you are going to annoy everyone who is reading your entry on a small monitor. Do not annoy your readers.
Quite simply: if your audition is more than one page long include navigation on every single page of your entry. It doesn't have to be fancy, a next and previous button will suffice. Just do it.
A note on why you should care about OCTs and why you should not:
There are many reasons why OCTs are good for a community that is interested in visual storytelling. If you're joining an OCT because you want to explore the possibilities of your character, improve your comic or flash skills, practice improvisational writing, learn to draw more dynamic work, or participate in a community exercise than you are doing it for the right reasons. Getting to see your character in the style of other talented artists and writers is pretty excellent too.
A lot of people join or create OCTs to show off, win prizes, be a pain in the ass, or become famous. Creating an OCT to get attention is one of the worst reasons to be a host and has been shown time and again to only make people more jaded and disappointed by the OCT scene. Stop ruining it for everyone else.
OCTs are amazing when they promote friendly competition and help people improve themselves. They bring people together as a community and can produce really amazing stories if you give them a chance.
In summary, the best auditions are the most unique ones. If you are weak in some areas of storytelling or design, then you need to focus on improving those, but if you're strong, flaunt your strengths. The auditions that will ultimately make it into a tournament will have exemplary moments of storytelling, narrative, dynamic action, or unique characters.