How much do I have to use Ogre?

I'm a little lost here, I don't know what exactly NxOgre wraps from Ogre.
I have a project in which I made a menu with Ogre, I want to start using NxOgre when I hit the button "New Match" since PhysX is not necessary in the menu. I don't know if this is posible because when I start using NxOgre I have to create a World which, as I understand it, it will create another Ogre::root, but I've already created a root in the Menu.
Also I don't know if I have to create things in Ogre such as cameras, viewports, entities, etc... or I can to do it with NxOgre.
Thanks for the help.

No, NxOgre doesn't really wrap Ogre. It just works with some parts of it.

World doesn't create Root, you do. You still have to do cameras,viewports, and so on. NxOgre can create entities/scenenodes but to the things that it's physically simulating.

Ok, I understand now, I will only use NxOgre with the things that have physics.
So I can start using Ogre once I hit "New Match". I create a World and I just start using it, right?
One other thing, in the Simple Sample app, there is this:
void chooseSceneManager(void)
{
mSceneMgr = mRoot->createSceneManager(ST_EXTERIOR_CLOSE);
}
and later:
mNxScene = mNxWorld->createScene("NxOgreScene", mSceneMgr,"gravity:yes, floor:yes, renderer:ogre");
In the case of Scenes I have to create them not only with NxOgre but also with Ogre, right?

Yep. But in Ogre they are called SceneManager

Thank you very much!