You shoud just view Osta as a "cycle", especially when running 2-3 caps/day. Obviously, there is no liver stress with Osta and very few side effects, so there is little to worry about with its use, but since it is still suppressive at the dosages used by BB'rs, you should consider it a "cycle".
Therefore, whether you are running Osta and then immdiately transitioning into steroids...or the other way around, it is just one extended cycle. Whether you are running an 8 week, 12 week, or 16 week cycle--I don't think it really matters. All are cycles...and all can potentially reduce T levels down to just about nothing, depending on what and how much is being used.
We can't really say how long a cycle can be...or how many cycles one can run, before it will permanently negatuvely affect natural T levels. Most guys, after running several cycles, will experience at least some degree of long-term supression, which can range from very minor to significant. People are different. If someone has T level of only 350 before they run their 1st cycle, they may push their natural prpoduction into a deficient range after just a couple cycles, since they started out so close to being test deficient. On the other hand, someone who has a T level of 1,200 before starting (it happens) is likely going to be able to maintain a normal T level even after many years of cycling...because even if his natural T production drops, he is more likely to end up in a decent range.
We also need to consider the fact that some people simply recover easier than others. Obviously, this will make a difference in the long-run. I knew one guy who used steroids ON & OFF for over 10 years in the 80's (a doctor)...never used PCT a day in his life...and even at 55 years old, over 20 years after running his last cycle, his T level is over 1,000. Not bad for a 55 year old guy who used steroid for 10+ years! I have also seen guys who ran only a couple cycle and required life-long TRT. These are the guys who had a low natural T level to begin with and had a hard time recovering--the worst possible combination.
So, I can't give you a single answer, as everyone will respond differently even when doing the exact same thing. I don't think anyone who runs steroids for several yeas, even when using PCT, is going to escape unblemished. Even in the best case scenarios, east some degree of long-term suppression is ging to take place.