HI mjpd, welcome to the board
. You're pretty young to have osteo, but the good news is that you have time to turn things around. An eating disorder is rough on bones. When you don't get the proper nutrients (calcium, ect.) from food, your body takes it from the one place it can get it, your bones. Your bones have been supporting you calcium needs resulting in osteoporosis. The first thing you'd probably have to do is get that under control if it already isn't and get going on a healthier lifestyle. Talk to your doctor and see if he/she can get you started on a program that will help you achieve better bones. Calcium supplements or any other bone building supplement aren't going to make you fat. But eating a diet rich in calcium from dairy and non dairy sources, fruits, veggies, and lean protein could put weight on you. Since you are asking for advice i hope that means you're willing to change to make your bones and health better. Sorry to be so blunt, but you since had the courage to tell us about the anorexia, i feel you want honesty.
You ask about exercise. The best exercise for your bones are those that are weight bearing like walking, jogging, jumping, anything that impacts your foot on landing on the ground. Weight training is also good since its the pull of the muscle on the bone that makes them strong. You can look it up online or talk to a trainer, who is familiar with osteo, or physical therapist.
Something else you might want to do is read the book the Myth of Osteoporosis by Gillian Sanson. It has a lot of good info and will answer some of your questions and help with the fear factor. If you have more questions please ask, there are plenty of very helpful people here who will be glad to help. take care..phyllis