If you are being interviewed for a job teaching English you must be fairly proficient in the language. My first advice to you, then, is not to be nervous. Will it be your first class? If so, you can begin with some simple phrases, such as "Good morning. My name is ..." I do believe, however, if you are not the least bit nervous your class will flow naturally.

I am not sure what type of oral lesson you have to prepare.A debate, oral presentation, oral book review? Italki is a great resource for English teachers too. Click onto English at Language Index at the top which will give you lots of links & hints. Go up to Resources & look at onto How to speak English like a native speaker, for some hints. Don't forget to visit the sections; Resources, Files, Knowledge & Groups at the top of this page too, maybe you can find something there as well. I wish you every success at your interview!