Strengthening teeth is a broad concept. If you're discussing specifically to preventing cavities, then chewing xylitol gum or using fluoride mouth rinses and such can help prevent cavities through different methods.
Osteoporosis probably won't affect the density of your teeth too much but may affect your jawbone. The outer enamel layer of a tooth doesn't repair itself as there isn't any live cells to produce more enamel. So any problem with osteoporosis shouldn't affect the outer enamel layer.