I am not familiar with OsteoStrong specifically, but have had lots of PT over the years. In my experience, there can be either "good pain" or "bad pain " afterwards.

Since you are often activating muscles and tendons that have become weak, stiff or lazy, some pain ( the good kind) is to be expected. The therapist should warn you what to expect. It should respond to heat, ice or a mild pain reliever. If not it's time to call the therapist.

Bad pain can happen when you try to do too much too soon, the therapist uses a cookie cutter approach and doesn't adjust equipment or intensity to you specifically, the exercises are done wrong, or you have underlying conditions that make it the wrong thing for you.

When I have had issues, first I talk to the therapist. If not satisfied, I will change people or programs. Finally, did you clear your fitness for the program with your docs?
Sue

I’ve been doing osteostrong for 3 months now, can’t sleep on my back anymore. Just now wondering if it’s the one machine where you press against your back. What happened to your back pain?