Help needed for Axapta data migration

You've got a tough job.

Which version of Dynamics are you running on Oracle?

What tables have been modified in Dynamics?

What I would do:
Clean install of Dynamics Ax 4 SP1 on a test server
upgrade to SP2 + latest DIS layer for your country
Create a project for your "data" related changes
(new base enums, extended datatypes, table modifications)
after making all the changes verify the new SQL server tables have the same fields as the Oracle tables.
Here comes the tricky part
Create a job in in your datamigration project that establishes an ODBC connection to the Oracle database. Create a query to open one of the tables (you will need to create a seperate job for each of the tables you want to migrate). Map the fields returned from the query to the fields in your new application table. call the validate write method on the destination table and if all is well then call the write method.

Its not trivial, its not the only way - you do need to watch for right justified text fields and modify them to be left justified before inserting them in the destination table. Read all of the upgrade documentation you can find and understand it.