I'd say when Jim Harbaugh took 41 point underdog Stanford and defeated #1 USC in 2007, he "unseated" Pete Carroll. He had a bad team but he got a great win over them. He lost to them in 2008 and then hung 55 points on them in a blowout in 2009. He beat USC again in 2010 when they went 12-1 and won the Orange Bowl.
So when Harbaugh took over 1-11 Stanford and USC was at the absolute peak of all CFB, he went 3-1 against them in his four years including scoring the most points USC had ever allowed in a game and this was with him and Pete Carroll having some public war of words and getting USC's full attention.
Taking that into consideration, when somebody says that OSU is simply on another level and Harbaugh has no chance against them, let's just say he's faced longer odds before and overcome them.