What are you trying to do exactly? The OJS database is designed to be run from a more server-level database (eg. MySQL, SQL Server, etc).
Are you just trying to generate a diagram of the database schema (ie. a Chen/Martin ERD or Booch diagram)? If this is the case, you might do best to get a CASE tool that does database reverse-engineering.
As for documentation, there is a the OJS Technical Reference (http://pkp.sfu.ca/ojs/OJSTechnicalReference.pdf
), which describes overall design and development details, but not data flows.
We might be able to help better if you can be more specific about what you're looking to do.