Put it simply, ‘O’ Sensei means ‘great teacher’ in loose translation. So effectively, we can address any of our teachers ‘O’ sensei, if we like to.
That begs the argument, why is there only one ‘O’ sensei?
Those who knows, will know that ‘O’ sensei generally refers to the founder to Aikido. But has anyone wondered why is ‘O’ sensei called ‘O’ sensei in the first place to begin with? Why wasn’t the founder of Modern Judo, another great teacher Jigoro Kano, known as ‘O’ sensei to his art and his followers? Or founders of other martial arts?
And who actually started addressing Ueshiba Morihei as ‘O’ sensei?