Wow.  T-shirts for the S&M crowd.  According to Webster, OBEY (oh-bay) is to comply with or follow the commands, restrictions, wishes, or instructions.  So if one is into bondage as Master / Slave then I guess the Master would be wearing these shirts, telling their partner to obey him.  Maybe they are also for parents with unruly children.  The possibilities abound.  I know this is a national brand (that sells some cool shirts and other things) and I'll bet they pronounce it oh-bee, but taken in the direct form of the word, it can be fun.