I am not sure what you are talking about regarding osteoporosis, nosferatu.
Exercise strengthens bones, thus reducing the risk of osteoporosis. If you already have medium to advanced osteoporosis, you would need to be very careful and only do certain kinds of exercise, but for a person in average condition exercise will only strengthen the bones.
As far as I am aware, exercise in moderation has no drawbacks, aside from the very occasional injury, such as twisting the ankle while jogging. Weight lifting, if done improperly, can be risky, that is why one needs initial instruction and supervision.
If one has a physical condition then they need to start slowly. And, some asthmatics have breathing difficulty when exercising.
Over-exercising could cause stress and diminished immune system, but that would be like 3 hours or more daily.
Some people run 20 miles almost every day and they are in great health, so over-exercise is not an issue for most people, except athletes.
Generally speaking, walking and swimming are safe things that pretty much anyone can do, reaping the benefits without worrying about any problems. 30 minutes a day (working up to this, if need be) is a good amount.
The worst effect of exercise is that it makes you sweaty, so be sure to take a shower afterwards :-)