The singular takes o-stem or s-stem endings, the dual takes mostly i-stem endings, and the plural takes s-stem endings. Halla-aho considers the dual forms a remnant of an original i-stem which later became an s-stem, but Derksen reconstructs the Proto-Slavic form as an o-stem instead.
Cejtlin, R.M.; Večerka, R.; Blagova, E., editors (1994), “око”, inСтарославянский словарь (по рукописям X—XI веков) [Old Church Slavonic Dictionary (Based on 10–11th Century Manuscripts)] (overall work in Russian), Moscow: Russkij jazyk
Derksen, Rick (2008)Etymological Dictionary of the Slavic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 4), Leiden, Boston: Brill,→ISBN
Halla-aho, Jussi (2006)Problems of Proto-Slavic Historical Nominal Morphology: On the Basis of Old Church Slavic (Slavica Helsingiensia;26), Helsinki: University of Helsinki
Sreznevsky, Izmail I. (1902) “око”, inМатеріалы для Словаря древне-русскаго языка по письменнымъ памятникамъ [Materials for the Dictionary of the Old East Slavic Language Based on Written Monuments][4] (in Russian), volume 2(Л – П), Saint Petersburg: Department of Russian Language and Literature of the Imperial Academy of Sciences,column642
Tymchenko, E. K., editor (1930), “воко; око”, inІсторичний словник українського язика [Historical Dictionary of the Ukrainian Language] (in Ukrainian), volume 1, number 1(А – Г), Kharkiv, Kyiv: State Publishing House of Ukraine, page294
Hrynchyshyn, D. H., editor (1978), “*око¹”, inСловник староукраїнської мови XIV–XV ст. [Dictionary of the Old Ukrainian Language of the 14ᵗʰ–15ᵗʰ cc.] (in Ukrainian), volume 2(Н – Ѳ), Kyiv: Naukova Dumka, page78
Bulyka, A. M., editor (2002), “око, воко”, inГістарычны слоўнік беларускай мовы [Historical Dictionary of the Belarusian Language] (in Belarusian), numbers 22(оддыханье – ость), Minsk: Belaruskaia navuka,→ISBN,page136
Hrynchyshyn, D. H., editor (1997), “воко; око”, inСловник української мови XVI – 1-ї пол. XVII ст. [Dictionary of the Ukrainian Language of 16ᵗʰ – 1ˢᵗ half of 17ᵗʰ c.] (in Ukrainian), numbers 4(весь – вправѣ), Lviv:KIUS,→ISBN, page197
Tymchenko, E. K. (2003) “око”, inNimchuk, V. V., editor,Матеріали до словника писемної та книжної української мови XV–XVIII ст. [Materials for the Dictionary of the Written and Book Ukrainian Language of 15ᵗʰ–18ᵗʰ cc.] (in Ukrainian), volumes 2(О – Я), Kyiv, New York: National Academy of Sciences of Ukraine, Ukrainian Academy of Arts and Sciences in the U.S.,→ISBN, page32
Vasmer, Max (1964–1973) “око”, inOleg Trubachyov, transl.,Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress
Chernykh, P. Ja. (1999) “око”, inИсторико-этимологический словарь русского языка [Historical-Etymological Dictionary of the Russian Language] (in Russian), 3rd edition, volume 1(а – пантомима), Moscow: Russian Lang.,→ISBN, page594
Melnychuk, O. S., editor (2003), “о́ко”, inЕтимологічний словник української мови [Etymological Dictionary of the Ukrainian Language] (in Ukrainian), volume 4(Н – П), Kyiv: Naukova Dumka,→ISBN,page170
Shyrokov, V. A., editor (2010–2025), “око”, inСловник української мови: у 20 т. [Dictionary of the Ukrainian Language: in 20 vols] (in Ukrainian), volumes 1–15(а – п'ять), Kyiv: Naukova Dumka; Ukrainian Lingua-Information Fund,→ISBN
