First I would like to welcome you to SWN Kay. OCC is such an amazing ministry. My pastor's wife, who is also our Women's Ministry leader, found the following website and asked me to organize some ladies to make some.
You don't need to buy a pattern. You can follow this tutorial and I'm also in the midst of making one to share here on this website and with women in my church. Mine will be finished by the end of the weekend.
I think I would begin by making a pillowcase dress and showing it to someone in your church, like the Women's ministry. If your church has participated in the OCC in the past, this should be easy to pull off. If you need more help, please let me know.
Be on the lookout for my tutorial, but I'm pretty sure you won't need it. Good luck and please keep me posted! I'd love to hear about the progress.