You want to avoid compression force on the vertebrae. Especially on the anterior portions of the vertebrae. Things like crunches and sit ups are examples of these.
The key to exercise with the goal of slowing bone loss or improving bone density is to create tension on the attachment sites of the muscles and ligaments. It is very similar to any exercise progression. But the degree of osteoporosis is very important. You don’t need a lot of tension to get a change in bone density. But it does need to be greater than the person is used to placing on the connective tissues of the skeletal system. And it needs to be applied regularly. You can’t exercise once a week and get more bone density. And physician clearance is absolutely necessary.