There are a lot of differences between osteoporosis & osteomalacia & the following are a few of them-
(1) Osteoporosis refers to the degeneration of already constructed bone. Osteomalacia on the other hand is an abnormality in the building process of bone.
(2) Osteoporosis means thin bone & the term implies a reduction in bone mass including all components of bone and not just calcium. Osteomalacia on the other hand is a metabolic disorder due to deficiency of Vitamin D leading to low level of Calcium & Phosphate in plasma causing failure of calcification of bones.
(3) Osteoporosis results due to the imbalance between destruction & creation of bone tissue, thus here is a negative balance & so the body can not create enough new bone to replace the already destroyed bone, though the building process was normal before. Osteomalacia on the other hand results when the building process of bone becomes abnormal from a deficiency in calcium and phosphate, usually due to inadequate levels of vitamin D or sunlight.
(4) In osteoporosis mineralisation remains normal but the bone mass is reduced & so the bone becomes brittle. In osteomalacia on the other hand mineralisation decreases & the bone becomes soft though the bone mass may be variable.
(5) Osteoporosis generally affects elderly and postmenopausal women whereas osteomalacia may occur at any age in adults. When the disorder occurs in children it is called rickets but in adults it is called osteomalacia.
(6) In osteoporosis the aetiological causes may include idiopathic, endocrine disorders, inactivity, disuse, excess alcohol, chronic renal failure etc whereas in osteomalacia the causes include vitamin D deficiency, abnormality of vitamin D pathway, non-exposure to sun light due to immobility or any other reason, malabsorption after gastric surgery or due to celiac disease, renal tubular acidosis etc.
(7) In osteoporosis the pain is referable to the site of lesion whereas in osteomalacia there is generalised bone pain.
(8) In osteoporosis the tenderness is elicited at the site of fracture, if any whereas in osteomalacia there is both generalised tenderness & tenderness at the fracture site.
(9) In osteomalacia as well as in rickets, both calcium & phosphate are normal or may be lowerd whereas in osteoporosis, both calcium & phosphate are normal.
(10) In osteomalacia alkaline phosphatase is always elevated whereas in osteoporosis, alkaline phosphatase is always normal.
(11) Osteoporosis is usually treated with various medications which can help to slow the progression of the bone deterioration. Since osteomalacia is usually caused by a dietary or sunlight deficiency, getting extra doses of these two elements may play a vital role in improving the condition.
But the risk for developing both conditions can be reduced by getting plenty of calcium and vitamin D in the diet.

Osteoporosis is the loss of bone density or the thinning of bone tissue, whereas Ostomalacia is the softening bones (due to lack of Vitamin D).