Proto-Indo-European Roots

Root/Stem: *ost-, *kost-
Meanings:  a bone, a rib
Cognates ():
Hellenic Greek osteon, Attic ostón 'bone'
Italic Latin os, gen. ossis 'bone', Old Latin ossum, nom. pl. ossua
Latin costa 'a rib'
Celtic Welsh asgwrn 'bones', Cornish ascorn, Breton ascourn
Indic Sanskrit asthi- 'a bone', gen. asthnas
Dardic & Nuristani
Iranian Avestan ast, asti- 'bone'
Anatolian Hittite haštai-, hesta- 'bones'
Tocharian Tocharian B ásta 'bones'
Armenian Armenian oskr 'bone'
Balkan Albanian ašt, ašte 'bones'
Slavic Common Slavic *osti, *kosti 'bone' >
Russian kost' 'bone', ostov 'skeleton'
Notes: The two variants of the root with the same meaning are very interesting, especially in Italic and Slavic where they are used parallelly. The newest research makes some linguists (Gamkrelidze & Ivanov) think that this dualism is a result of some palatal phoneme in initial position. 
The idea that there was a laryngeal instead of *k- originally was supported by the Hittite forms, but today the majority of specialists do not maintain it.