It's a Japanese cultural thing. It has nothing to do with his English ability. A lot of Japanese people do that to indicate that they're listening when you're talking to them or that they understand something you said. You'll hear similar things like "sou desu ne" or "hai" inserted in too. Think of when people in English say things like "uh huh" and such when you are speaking to them.
I never really noticed the ohs, but I make hmm noices when I see something interesting.
BTW, Lin. What's that little jingle at the end of the videos called?