Hello Rajesh,
As all of this is rather theoretical for me (I don't have a lot of experience using Oracle with DNN), I can only say the following: as DNN works with a provider system for its DB connections, it should still work PROVIDED that these providers are compatible between the two different DB systems;
I would recommend the following steps:
1. backup your full site (files & database)
2. restore the database in a test environment + copy all the files + point the connections string in the web.config towards the non-production database
Now you have a safe testing sandbox which you should be able to use. In other forums / blogs you might find more detailed information about how to backup and restore a DNN site locally. Please also note that if you're going to port this to another (test) domain / localhost you'll have to configure the portal aliases; if you forget to do this you'll have to put them into your database tables by hand (table PortalAliases).
Next you'll migrate that local version using SSMA. Normally that will not really alter your Oracle database (just read it) but it's always safest not to use your production environment for these kinds of tests until we are sure this will work. At the end, you'll have a MSSQL database version of your database.
Only thing you should do to test is point the web.config's connections strings to the newly migrated MSSQL database. Host up your test website and check if it still serves up pages.
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