The most powerful class of painkillers is the class of opioids - the most famous of which is "oxycodone" / "oxycontin." So when B (I think the last A should be a B, since this is a conversation) said "the kid stuff, nothing with an 'o'", he is saying that the doctor only gave him a less powerful painkiller, and the name of the drug does not start with an "o" (like oxycodone/oxycontin).