Let me help you out. :3

Basically, OCTs are a bit like artist tournaments.

First, you create an original character as your stand-in in the tournament (they don't have to be anything like you, though - they mostly serve to showcase your writing and design abilities). With this character, the artists (and sometimes writers and animators) enter the tournament, often by auditioning with a small comic or short piece of writing for one of a limited amount of free slots.

As soon as the participants are chosen by the judges of the tournament, pairs of them will be determined who face off each other. To do that, both opponents each create a piece of work in which their character in some way, shape or form encounters and ideally overpowers the character of the opponent. After the deadline for these pieces of work, called "Entries" has passed, the judges will try and determine which entry of every pair was the more creative, entertaining and well executed one, and it's creator will win the duel and move on to the next round.

Usually, OCTs have overarching plotlines and a predetermined setting that the entries take place in, as well as a canon just like any series or story. The interesting thing is that the winners of the battles influence the canon story with their entries - when you win a battle, the comic, animation or piece of writing that you created becomes what canonically happened at this point in time in the story of the tournament. This can lead to very interesting plot twists and to participants influencing each other even without direct interaction - for example, if an arena is destroeyed in the canon story of a winner, the next artists who want to set their fight in their might depict it as a bunch of ruins - and might use that to their advantage in their entries.

There are several goals OCTs are trying to achieve:
- they provide an opportunity to get plenty of practice for the participants
- they try to achieve an interesting overall story
- they train and encourage creativity and design and writing prowess in their participants
- they help getting used to working with deadlines (which everyone who wants to do art, animation or writing for a living needs to learn)
- and they help getting used to narrative media (for example: animators who only created small gifs of their characters standing still and breathing learn how to animate a coherent story, artists learn how to draw comics, etc)
- also, of course, they exist to let everyone participating in and watching them have fun! :3

I hope this was coherent and helpful. ^u^ I'd love to face off against you in an OCT at some point in time. ;3 Have a nice day!