Board Thread:Theories and Speculation/@comment-45444155-20200728155230/@comment-32685487-20200729175521

You have to balance the natural energy and chakra together perfectly to make Senjutsu, that is also how you enter Sage Mode. Orochimaru wasn't eaten by the White Snake Sage, so that means he successfully entered Sage Mode. The problem is his host body could not handle Sage Mode therefor Oro does not use Sage Mode except for moulding Senjutsu which he apparently exits Sage Mode as soon as he finishes doing so. Orochimaru created the Cursed Mark with his own Senjutsu, and Jugo's enzymes and DNA.