If you are using Visual Studio 4.0 (not sure about 3.5) and
you have enough right to access the database, you could use the Entity Framework for it. It is discribed on this page Use Entity Framework with Oracle Data Provider for .NET
]. Generate a model from the database and you have a database diagram you could use.
While searching for a free tool I found this one http://www.oracle.com/technetwork/developer-tools/datamodeler/overview/index.html
]. I looks like it is possible to import a database. At least there is a demo about it. You could Google to search for other tools.
I'm not sure what you could do when there is some reason that you can't reach the database. Analysing it manually could be a last resort. Draw the diagram yourself in a tool like Visio or use good old pencil and paper :)
Hopefully the tool or entity framework could be of help.