Final answer:
Oxygen (O) is different from sodium (Na), magnesium (Mg), and osmium (Os) because it is a nonmetal with unique chemical and physical properties compared to these metals.
Step-by-step explanation:
Among the elements listed - oxygen (O), sodium (Na), magnesium (Mg), and osmium (Os) - the element that has properties different from the rest is oxygen (O). Oxygen is a nonmetal that exists as a diatomic molecule (O2) under standard conditions and is essential for respiration in many organisms. In contrast, sodium (Na), magnesium (Mg), and osmium (Os) are all metals. Sodium and magnesium are light metals that are highly reactive, particularly with water, while osmium is a dense, heavy metal that is much less reactive. The noticeable difference between oxygen and the other elements is that elements like sodium and magnesium are shiny, conduct heat and electricity well, and have certain chemical behaviors in common, whereas oxygen, being a nonmetal, has drastically different properties such as poor conductivity and a different chemical reactivity pattern.