@xXRaDsKiXx If you are talking about if you need to add something to show that you drew an OC for someone else and aren't just submitting another OC, then I believe you would need to add something to the description of the post explaining it, or just let the person that you are drawing the OC for post it themselves.
"Hello, Thanks for applying to have your OC flying in the community collab image. Unfortunately, your request was not approved due to the number of submissions we got. However you are still welcome to submit your OC in a standing or sitting pose, and we’d like to see what yo come up with! Joey"
This may be a silly question, but if a piece is submitted for the collab, is it allowed to be submitted as a stand-alone piece on other websites, like deviantART or the like? Or should it remain exclusive to derpibooru?