Oh Yeah was just a quick vocal session; one of the Boys (Carl, I think) met a young black kid and somehow got him into Brian's studio to do what was basically something between rapping and singing while the Boys minus Brian cooed "oh yeah!" in the background.
It was like this -
I got the spirit (oh yeah!)
I know you want it (oh yeah!)
But you can't have it (oh yeah!)
'Cause I got it (oh yeah!)
It's in my hands (oh yeah!)
I can't fight (oh yeah!)
It's in my eyes (oh yeah!)
I can't see (oh yeah!)
It's in my mouth (oh yeah!)
I can't talk (oh yeah!)
It's in my nose (oh yeah!)
I can't breathe (oh yeah!)
It's in my legs (oh yeah!)
I can't walk (oh yeah!)
It's in my oomph! (oh yeah!)
I can't oomph! (oh yeah!)
Oh yeah! (oh yeah!)
Oh yeah! (oh yeah!)
I don't think it was ever meant to be taken seriously, although the kid probably was dubbed a copy for his own amusement.
Not Brian's home studio - recorded October 14th 1968 at Capitol's New York studio (the band were there to do the Ed Sullivan Show).