@debbik It depends on how you define osteoporosis and how it's diagnosed. If you are using bone mineral density to determine it then many many people have moved from osteoporosis (-2.5 t-score or worse) to osteopenia and even up to "normal" levels. That's usually with medication. Of course if you were diagnosed because you had fragility fractures you can't undo fractures but you can still improve bone density above -2.5.

Also Belinda Beck and her LIFTMOR and related studies had people go from osteoporosis to osteopenia with heavy exercise alone. They consistently got significant improvement in bone density with their protocol.

As to whether the bone can ever be as strong as it was before bone loss I have not seen an answer to that. It is clear however that you can increase bone density so that the label osteoporosis based on DXA score no longer applies. It's also clear that fractures are reduced. So whether it gets as strong as before is not known but it's very clear if you improve bone density your bones are stronger and less likely to fracture. And of course it feels very good when those numbers improve.

BTW, that brief article (not a study) someone mentioned saying you cannot reverse osteoporosis was pretty much useless, IMO.

I went from severe osteoporosis in my LS with a worse score of -4.1 to a worst score in my LS of -2.1. So that's severe osteoporosis to osteopenia in less than a year. My hips didn't make it though as I've still got a -2.5 in my femoral neck. Hopefully next DXA my FN will be out of the osteoporosis numerical range and be stronger.
That was done with Evenity and I also followed a modified LIFTMOR protocol. With continued good fortune I hope to be more improved over the next couple of years. Fingers crossed.