How to detect which database the application should connect?

my application will run on premisses and should allow by users to select between oracle and sql server databases.

I have the classes to Oracle and SQL Server with the commands ready and my question is how to make it happens during the execution of the app.

I really think it could be done during the application start (IIS). The app should check a info on the web.config and goes to dll it wants to (sql or oracle dal).

How can I do that? What is the best approach to do it? There is designs patterns I should understand to make it easier?

ps: EF and any other ORM are an option on this scenario.

Thanks in advance

EDIT

In time, the user should be allowed to change the database (oracle to sql server or sql server to oracle) after installed just changing the config and reseting the iis.