This wiki follows the same guidelines as to copyrights that are explained in detail by Wikipedia. It's quite a read. The gist of that material is: if you contribute to the wiki, you don't own your work. It falls under the GNU Free Documentation License (GFDL), which basically says that anything under that license can be reproduced and modified without prior consent.
It's important to note here that this wiki is primarily about a game that is owned by Take Two Interactive. Therefore, any content from that game which is posted here is done so with their implicit permission (i.e., they haven't asked us to take it down). That includes information about the inner workings of the game, as well as art assets and story details. So that material is not in fact under the GFDL, only your own words are. In fact, the line separating Oni's copyrighted content from the wiki's contributed content about Oni may be impossible to draw without a legal expert, so you should not assume that you are familiar with the legal status of any content on this wiki, no matter its source.
Okay, enough of the formal legal junk. Does this actually mean anything you should be concerned about? Only if you post fiction here which you have written and hope to do something with, at some point in the future. If you don't plan to make money off it, then maybe you're okay with it being under the GFDL, but certainly any plans for commercially publishing games or stories could be legally endangered by placing them here. You might be able to protect them by explicitly declaring a copyright of your own, but don't risk it. It's better to be safe than sorry. Of course, if your work is here, it's probably Oni-related, so it may also be subject to Take Two's copyright on the game. Caveat Contributor, in short.
The official legal language found in the GFDL overrides the preceding text in the event of any inaccuracies in that preceding text. Be sure to read the GFDL if this copyright issue is of serious concern to you.