Best solution for accessing multiple databases?

Is there one Oracle DB or are there several and which one needs to be
determined based on the user or some other criteria? The article you
link to talks about the latter.

It it's just one DB and you have one or more tables to be 'statically'
connected to that DB , then see and look
for the section "Connection to multiple databases in different models"

You probably want to call establish_connection with the right
connection parameters for the table(s) to be connected to the oracle
DB in your config/environment.rb file in a config.after_initialize
block. If you've got several tables you can make an abstract AR class
for those models to subclass, and establish the connection for the
abstract class which will take care of the subclass.

I'm working on a project right now which does something similar. One
thing I've found is that most of the testing infrastructure of rails
doesn't really handle tables with their own connections very well,
what I've done is to put analogs of those tables directly in the main
test environment database. I wrote the migrations for the 'external'
tables so that they only actually create the tables if
ENV['RAILS_ENV'] is 'test'