You’ll need to do a full code review to ensure that Oracle even supports all the features that you are using. Any unsupported features you’ll need to find a way around.
Then you’ll need to script out the schema, switch all the data types from the SQL Server data types, to the Oracle data types, and import some data for testing.
Then you’ll need to rewrite all the stored procedures in PL/SQL (this includes any inline code within the application). Don’t forget to change all your variables to the Oracle data types.
Then you’ll want to rewrite any large batch operations as row by row cursors so that they perform better in Oracle (Oracle does better with row by row operations, where SQL Server does better with batch operations).
This will take months (if not longer) and will probably cost you a fortune. What’s the driving factor to move from Microsoft SQL Server to Oracle?