dancingalone
Grandmaster
Ah, but one of the reasons that Ueshiba Morihei was given that title wasn't necessarily to do with his skills, it was because both he and his son, Ueshiba Kisshomaru, were instructors for the same student body. In order to differentiate which Ueshiba Sensei had given some instruction, or advice, the elder Ueshiba was referred to as "O Sensei", referring to him as the elder, or "bigger" of the two. It has since become more part of the mythos of Ueshiba, rather than being part of the original use.
Ueshiba Kisshomaru was referred to as Waka Sensei or 'young teacher' to differentiate him from his father. To my knowledge, once he became the second doshu he never used or was referred to as O'Sensei, which seemingly argues that it was a singular title reserved for his father.
I can see the point you are making. Perhaps O'Sensei did start as a mere way of distinguishing the father and son from each other. However today, O'Sensei means Ueshiba Morihei Sensei specifically.