ObBodMod stands for "Obligatory Body Modification", and is used in the context of an online posting in a body modification related forum.
Newsgroups such as rec.arts.bodyart and bodmod mailing lists typically expect the postings to be on-topic. That is, about body modification. However, often the topics stray into other matters. Posts are "justified" by what amounts to a "PS" — At the end of the message you may see something like:
ObBodMod: I got my ears pierced today. I screamed like a stuck pig and my piercer won't let me come back.