Board Thread:Naruto Discussions/@comment-30168130-20161009021213/@comment-4912646-20161009231401

The whole idea of using senjutsu without Sage Mode/Transformation seems inconsistent with what we saw during Naruto's training, but it's always been there. Even back in the Pain arc, there was evidence that it's possible to use (or at least build up) senjutsu chakra without entering Sage Mode, since the shadow clones Naruto left on standby to gather natural energy didn't display any sign of actually being in Sage Mode. We see the same thing when Madara absorbed Hashirama's senjutsu chakra, despite the latter being in his regular form at the time.

Maybe when you're first learning to create senjutsu chakra and can't properly control it, you automatically enter Sage Mode like Naruto did during his training, but once you've learned to handle it better you can build it up and intentionally keep it on standby, so as not to expend it unnecessarily. That could also explain some of those occasions where characters seem to enter Sage Mode instantly without having to sit still and gather energy beforehand.

As for Orochimaru, it would help to know why is body is/was incapable of using Sage Mode in the first place. All we were told back in the Pain arc is that you need an enormous amount of chakra for it to work, so maybe his stamina just wasn't good enough? The old databooks did rank it (surprisingly) low in comparison to monsters like Jiraiya and Naruto.