Hi, Michelle. The short answer is bipolar is considered an episodic mood disorder distinguished by cycles from depression to mania or hypomania (light mania), while schizophrenia is considered a chronic thought disorder distinguished by psychosis (such as hallucinations and irrational beliefs) and deficits in mental processing.

In reality, there is considerable overlap between the two illnesses. For instance, most bipolars who experience mania also experience at least some symptoms of psychosis, plus many experience difficulties thinking clearly, even with no mood symptoms. Also, people with schizophrenia may suffer from depression.

There is a diagnosis called schizoaffective disorder, which can be thought of as a hybrid bipolar-schizophrenia diagnosis. There is a lot of controversy surrounding this diagnosis. The main thing to look for - it seems - is full psychosis occuring at the same time or in close proximity to full mania.

As for OCD, that is considered an anxiety disorder. Fear governs anxiety, and anxiety may trigger a mood episode or even psychosis. So managing your OCD is a way to manage your depression and mania and psychosis (but you will also need to have a lot of other managment tools in place as well - meds, sleep, lifestyle, support, etc).

Hope this helps -