Attached in this post is a copy of the certificate that Takeda Sokaku issued to Ueshiba Morihei in the art of Daito Ryu Aikijujutsu.
I have, however, marked in red a section that reads "Takeda O'Sensei".
Now, I thought that only Ueshiba was reffered to as "O'Sensei", but it seems that I was wrong. At least his teacher, Takeda Sensei, was also using that title. And good to know too that he spelled it "大先生" (meaning Great Master), as opposed to "翁先生" (Venerable Master).
The 大先生 in Ueshiba's kyoju dairi is properly read "Dai Sensei". This is what Takeda was known by, as a check of some of the Daito Ryu related articles at Aikido Journal and Kondo Katsuyuki's website daito-ryu.org will confirm.
"Dai Sensei" is the usual reading of that compound. "O'Sensei" is somewhat irregular.
Thanks to Josh Reyer for the info.