Well long story short.....If you upload art/music to OGA it must be licensed under a creative commons license of your choosing so others can potentially include it in other projects. Each license has different conditions associated with it so you would have to choose the best one that suits you. For example CC0 has no requirements and others are free to use the asset however they wish without constraints whereas CC-BY-3.0 states that anyone who wants to use your art must attribute you in some way as the creator....There are a few others as well. Also submissions must not contain any sort of copyrighted material like logos, fan art stuff like that....
You can read up about the different licenses here: