I've seen the recent episode of the anime, and I saw when Obito summoned Kurama. I'm interested in the fact that does he get into some kind of "contract" with some animals/things? (That would mean that he is really able to summon.) I've checked on the Narutopedia, and it appears only the Gedo Mazo is attached to his name, so is the summoning of the Nine-tails just a normal/simple summoning technique? Because you must have that kind of bond between you and the summoned animal, which is made with your chakra/blood, just like Naruto did with the toads and it would be weird if you would be able to summon a tailed beast just "because".
From the the battles of Minato vs Obito and Hashirama Vs Madara I can recollect three important things:-
- Both Sharingan users[Obito and Madara] casted genjutsu to Kurama[Kyuubi] and as a result they were able to control it.Kurama was bound to obey them
- There was some contract seal between the caster and Kyuubi.
- Both Minato and Hashirama used their contract seal.
More about Contract Seal is explained here:-
Though the Contract Seal removes the summoner's ability to control its summon, it does not give the user of the fūinjutsu the ability to control the summon in turn.
From what I can speculate Obito got some information about the contract seal from Uchiha tablet or directly from Madara.
Of course, Obito mastered the Summoning Jutsu.The great disaster in Konoha was caused by Kurama and the one who summoned the Kyuubi was Obito Uchiha.
Tailed Beasts are not animals. They are large mass of chakra with there own consciousness. In the first fight between Hashirama and Madara, Madara could use Kyuubi like his summoning. That time Kyuubi was not bound within a jinchuriki. So I think someone as powerful as Madara could easily control the tailed beasts by overwriting their consciousness. And since Obito now had combined power of Uchiha as well as Senju, IMO he was strong enough to summon the tailed beast and use them.