Yes, even oral English teachers must do a proper lesson plan.

Oh why is that then- in case you forget what to teach? You know as well as I do that in Chinese middle schools and universities the same lesson(or 2) is taught to different classes all week. You must have a hundred lesson plans all in your head- don't you? A good teacher can go in cold. There may well be problems with that if the language you're going to teach is not your own language. However, the school or university will often require you to follow a text book, and all that requires you to do is to flit through it- the plan here is already done, you just need to mentally take note of a few things. Again not easy if you're a second language English teacher; so you will need to learn some English grammar yourself beforehand and write it down as a plan in case you forget. In China you'd better have some lesson plans in your head- you may well be sent in to teach classes you weren't expecting to teach. You can't skedaddle every time the Chinese spring a surprise on you.