Gummy, I dont think his snake form is his most powerful one, I believe he can only bring out the sword from his mouth in that form, anyways...when he is ready to transfer bodies, he comes out as a white snake; out of his host, and then enters another body. When he has come out of his former host, he is in a very weak state, and is out of fighting form...this was the state he was when Sasuke met him...so, Sasuke just killed him...I'm sure even Zabuza would have been able to kill Oro if he met him in that state.
I wish Kakashi wouildn't die...but I think he has to, to let Naruto be Hokage.