Is Tobi the same as Obito, disguising as Madara?
Now first of all, if you want to know whether Tobi is Obito, then there's a youtuber out there called '2spooky' and he has posted a video that clears that part.
Now to why Obito was referring to himself as Madara.
Madara's had the dream of creating the ideal world. But he won't be able to do that at his old age. So, in order to get resurrected with rinne rebirth jutsu, Madara implanted his rinnegan to newborn Nagato. But even so he still needed a successor to him who can persuade Nagato into to using rinne rebirth jutsu. And that successor was Obito.
But Obito didn't have a name for himself in the shinobi world. He is just a kid thought to be dead in the war. If he tried to live with his true name, he could do particularly nothing. But since Madara was a name known far and wide in the shinobi world, he went with that alias. With this alias he could persuade Nagato in turning akatsuki into a rouge ninja association, and he could strike fear into people.
Hope this helped.