From Greek osmē, smell (from the strong odor of osmium tetroxide). Named osmium after http://www.answers.com/topic/greek-language osme meaning "a smell", because of the smell of the volatile osmium tetroxide
Osmium has varying states of reactivity. Its pure state tends be quite reactive and is rarely used unless alloyed. Osmium tetroxide is extremely reactive, whereas osmium dioxide has very little reactivity.