Isn't it impossible to gain all the jutsu in the world? there are jutsu, like amateretsu and kamui, that depend on the sharingan eye. If oro choosees Obito, he misses out on amateretsu. He needs to find where Obito's Lab is (which he may know already) before taking over Obito.
Also, Oro just took over a zetsu. Before, the timer for the jutsu was 3 yrs (he couldn't use the jutsu for 3 yrs after using it). But, as we know, HS DNA screws with jutsus that have a long "cool down" timer. We know this when Itachi said to naruto ~if you had senju dna, the timer for suishi's MS abilities would decrease.
So, we don't know if Oro could take Obito's body soon.
who knows.