Proto-Indo-European Roots

Root/Stem: *oi-n-, *oi-k-, *oi-w-
Meanings:  one
Cognates:
Hellenic Greek oinos 'one'
Italic Latin únus 'one' >
Italian & Spanish uno, French & Romanian & Catalan un, Sardinian & Aromanian unu, Portuguese um
Celtic Old Irish oín, Welsh & Cornish un, Irish & Scottish aon, Breton unan
Indic Mitanni Aryan aika- 'one', Sanskrit & Pali eka- 'one', Prakrits ekko 'one'
Singhalese eka, Marathi & Nepali & Assamese & Hindi & Gujarati ek, Lahnda ikh, Punjabi yk
Dardic & Nuristani Kashmiri akh 'one', Dameli ek
Kati ev 'one', Kalasha ew
Iranian Avestan aêva- 'one', Old Persian aiva-,
Pahlavi évak, Khwarezmian 'yw, Ossetic yu, Pashto yau, Wakhi ji, Persian & Tadjik & Baluchi & Kurdish yak
Balkan Albanian nje", ni 'one'
Germanic Common Germanic *ainaz 'one' >
Gothic ains, Old English án, Old Frisian en, Old High German ein, Old Norse einn,
German ein, Scots ane, Norwegian & Swedish en, Icelandic einn, Dutch & Afrikaans & Low Saxon een, Frisian aan
Baltic Old Prussian ains 'one', Lithuanian vienas, Latvian viêns
Slavic Common Slavic *ed-inü 'one'
Notes: The root was obviously *oi-, with different nominal determinative suffixes which could be intechanged: Iranian and Nuristani *-w-, Indic and Dardic *-k-, other IE groups chose *-n-
The word *oinos in Proto-Indo-European was not a numeral, it was a typical adjective which could have feminine and neuter forms - and it remained adjective in all later Indo-European tongues. It was declined as any other substantive of o-stems.