I guess it's possible Nawaki might have had the potential to use the Mokuton, but he had barely ever seen battle before he died. How would Oro have known to look out for him in particular? Presumably, not even Hashirama's direct descendants (Nawaki and Tsunade's mom/dad, whichever was Hashirama's child) possessed the mokuton. Also, if Nawaki really did have the bloodline limit, why would Oro kill him? He could have took him under his wing like he did with Anko, while collecting information.
Even though the idea of Oro killing Nawaki is plausible, it's hard to fit such a fact into the story in a way that doesn't seem forced. After all, Tsunade's likely fighting her last battle right now, and story-wise, she'd have to discover this fact if it's going to mean anything for her character.