You can use "Strategies" in Designer, look at chapter 4 of the Designer's manual. A strategy is a script that reads structural information from a database or flat file, you'll need to create an external strategy, there is a prebuilt strategy for Oracle available.
You can also add your own modification to the SQL of the object in the SQL editor in Designer.
And finally you can force use of COST or RULE optimizations by Oracle in ORA.INIT or through an object in Designer.
Hope this helps Stick to your guns