an old thread.
As Peter indicated, it's used quite a lot in Yoshinkan but not at all in other Aikido organizations. In some Karate schools you will hear it a lot.
I have seen no evidence that is was ever commony used outside of Yoshinkan lineage schools.
Far be it from me to defend Homma Sensei, but what was quoted really is not far off from what I have heard from other native or fluent speakers of Japanese. In most contexts it's a way bit too informal.
My understanding is the origins of "Osu" is in crude informal usage among young males - around college age.
It's not hard to believe that particular group may have derived a very specialized flavor to the meaning in a particular context.