It comes from Bill Cosby, and was used in his stand up routine when he talked about going to the dentist. After having the injection his mouth has gone numb and his lips are hanging loosely. The dentist asks him how is the family and he says "ObKb", because he can't move his mouth normally.
There may also be other references to OBKB in Fat Albert and the Cosby Show.
Dentist "So Bill hows the family?"