My eight year old son was fine. One month ago, he gotsevere ocd. One doctor said "pandas" and gave something called an ivig. Other doctor said, "no such thing." what should I do?

It does happen. Paediatric autoimmune neuropsychiatric disorder associated with streptococcus infection does happen in a small number of children following strep infection. Exact mechanism still not known.Ivig is an experimental medication.
PANDAS is real.. The hallmark trait for pandas is sudden acute & debilitating onset of intense anxiety and mood lability accompanied by obsessive compulsive-like issues and/or tics in association with a streptococcal-a (gabhs) infection that has occurred immediately prior to the symptoms. Therefore, if this is an abrupt change in behavior, then pandas is possible. See http://pandasnetwork.Org to find a provider.
Controversy. PANDAS is still a debatable diagnosis, but there is reasonable evidence that such a disorder exists. IVIG has been one alternative for treatment. Has the OCD improved? If so, continue with the treatment.
Not decided yet. In our medical community, there is a lot of controversy. I've seen patients with neurologic symptoms who tested positive for strep and whose symptoms resolved with antibiotics, and others whose neurologic symptoms got better after finishing antibiotics. But symptoms came back a year or two later. It's worth doing a full course of anti-strep antibiotics, but it's not a 100% guarantee.
Debateable. This is a hotly debated topic. Pandas is a group of disorders (ocd, tourettes, etc) that develop rapidly and seem to be related to a recent streptococcal infection (such as strep throat). Treatment with IV IG is used sometimes...But is not proven. In fact, others debate that the infection has nothing to do with the symptoms and it is purely coincidental. The research continues...