High Potency Bone & Immune Support. The True Strength of ON Vitamin D: At ON, we care about quality and performance. And you deserve the quality support to help you achieve your optimal performance. Vitamin D helps promote healthy bones by the absorption of calcium, is good for immune system support, and plays an important role in general health. Taking extra vitamins can be just the support you need, so you can stay active, stay training, and stay in the game.