What would be the easiest way to migrate a database that I have in SQL Server 2005 to Oracle? I have around 6 tables each with a large amount of data in it.
Have a look at this Oracle page on Migrating Microsoft SQL Server to Oracle.
You could try the Database Publishing Wizard from Microsoft. It 'publishes' a database to a T-SQL script. I haven't tried it with migrating to Oracle directly. But, I suspect you could modify with the T-SQL script appropriately.
With only six tables, doing a manual conversion will probably get you the best result. Create an ERD, generate the Oracle schema, and then transfer the data by exporting CSV files and importing them. I've used SQL Developer as linked above, but it sometimes takes a bit of work to get everything working, and then you still have to create an ERD for future development. It did save me a lot of time on a conversion but that was over 100 tables. In that case it was faster to use the tool to get most of it created and then go fix everything up.