I am assuming you are using portal engine and the form is placed on a page using a web part. In this case, the best way would be adjusting the code of the web part - in the code you can check what are the values entered by the user and when submitting the form, the code could do the decision and redirection. The example in the Customizing web parts documentation is also using the online form web part - so you can follow those steps and just add your custom logic to the code.
Another idea would be maybe using some bit more complex macro expression in the mentioned "Redirect to URL" setting. This macro would be checking the desired field values and based on that, you will have some "IF-ELSE" statement and logic in the macro expression to do the trick.
If you are using MVC, then you can add some custom code on your MVC app to "listen" to the entered values and then, when submitting the form, do your custom action.
Other than that, there are no settings to do the decision logic based on what values are entered in the form.