Osiris wouldn't have been allowed to be present during twilight gap if he was an experiment, further more this patient b couldn't have been Osiris due to the fact that he was definitely present (and a major influence on the results) of the battle at twilight gap.
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Patient B died during the Golden Age, making him a possible candidate for revival. Also he's not so much an experiment as much as he is a test subject. He was fully human before death his demise was due to an experiment. One could say the Concordat were embellishing the truth a tad to further thier goals.