Tobirama was still Hokage when Hiruzen trained him so he probably knew of Orochimaru before then. And I agree with the part about Minato i thought that was awesome.Yep, being immortal has its advantages xd
Seriously though, the 1st and 2nd only know him through Edo Tensei. But it was cool that Minato looked at him out of respect.
I don't recall in anywhere being said that Tobirama was alive while Oro became Hiruzen's student. Proof from the manga?Tobirama was still Hokage when Hiruzen trained him so he probably knew of Orochimaru before then.