Proto-Indo-European Roots

Root/Stem: *okw-
Meanings:  an eye, to see
Cognates: Greek osse (an eye) - from *okwe > *ote according to Hellenic phonetic laws
Latin oculus (an eye) - masculine, o-stem noun, > 
French oeil, Spanish ojo, Portuguese olho, Romanian ochi, Sardinian okru, oju, Aromanian okli, Italian occhio, Catalan ull, Occitan uei
Common Celtic *okw- (to see), > 
Old Irish fuindeoc (a window) - from *wé- (to blow) + *okw-
Irish Gaelic fuinneog (a window)
Common Celtic *ehi-kwo- 'sight' >
Old Irish enech 'sight', Middle Welsh & Middle Breton enep
Hittite šakuwa (eyes, dual) - sometimes considered a prefixed noun from this very stem, but very doubtful
Tocharian A ak (an eye), Tocharian B ek
Sanskrit aks.i (an eye),  
Gypsy yak, Lahnda ekh, Nepali akho, Kashmiri achi, Hindi & Gujarati akh, Khaskura ankha, Punjabi ekkh
Armenian akn, gen. akan (an eye)
Common Germanic *augan (an eye), *agwian- (to see) > 
Gothic augo (an eye), Old English éage (an eye, neuter), Old Swedish óga, Old Frankish age, Old High German ouga, auga, Old Norse auga; Old Frisian auwa (to show);
English eye (an eye), Swedish oga, Afrikaans & Dutch oog, Frisian each, Faroese eyga, Danish oje, Norwegian oye, Icelandic auga, German Auge
Common Baltic *akis (an eye), > 
Old Prussian ackis (an eye), Lithuanian & Sudovian akis (feminine), Latvian acs
Common Slavic *oko (an eye, neuter), dual *oc'i, *oc'esa
Old Church Slavic & Old Russian & Ukrainian & Bulgarian & Czech & Polish & Slovene & Serbo-Croatian & Macedonian & Slovak oko, Upper Sorbian woko, Lower Sorbian hoko, Belorussian voka
Notes: This stem generated a verb "to see" and a noun "eye", of which the second appeared much more lively. We cannot say if the noun "eye" derived from this stem was masculine, feminine or neuter in Proto-Indo-European. Doubtless is only that it did not have plural but only dual forms. 
There is an opinion that this word for "eye" was in sacral use and meant "eyes of god". So many languages elaborated another word with the meaning "eye of human", like Greek ophthalmos or Slavic glaz. This may be a reason due to which this stem disappeared from a lot of IE tongues.