Board Thread:Naruto Discussions/@comment-4682609-20140806081616/@comment-26302001-20140806123352

because Obito's chakra mixed with Hashirama's for 18 years and essentially became his own, and I think it is safe to say Hashirama's DNA/Chakra makes a Mangekyo a makeshift EMS, therefor he was able to use the perfect Susanoo.