Sage Mode is an art that is extremely hard to perfect. It does grant the user an amazing boost, but only few characters have managed to use a perfect Sage Mode.
The Imperfect Sage Mode on the other hand, is easier to achieve. Here’s a list of all the Imperfect Sage Mode Users in Naruto —
Orochimaru was the first person to discover the Ryuchi Cave of the Snake Sage. It is unknown whether he mastered Sage Mode or not, but he was not able to use it.
That was because Sage Mode required vast Chakra reserves to begin with. But, he did devise a technique, called Cursed seals, that allowed others to feed on his Senjutsu Chakra after absorbing their own.