In general there's a utility that assists you with the migration of a SQL Server to Oracle - but this utility isn't able to read a SQL Server bak file directly.
So you first need to restore this BAK file into your SQL Server and then you can use this utility to migrate to Oracle.
You'll find all information on OTN:
Especially have a look at the Quick Start Guide => Migrating to Oracle Getting Started Guide
And here you'll find a Presentation, some Videos etc.:http://www.oracle.com/technetwork/database/migration/index-084442.html