I have the answer: (I used the OED)
Ox is the only word in general English use which retains the orig. plural ending -en (the reflex of Old English -an) of the weak declension. An older umlaut plural xen, exen occurs in Old English (Mercian and Northumbrian respectively), whence app. the Old and Middle English forms exen, exin, and Middle English exon. A new plural oxes, found from the 14th cent., has survived only in regional and non-standard use. The genitive singular oxes for oxan appears in Old English (Northumbrian) in Lindisf. Gospels (cf. quot. OE1 at sense 1a), and an -s genitive is normal from the Middle English period onwards.
A number of compounds of the base form of the word correspond to compounds of the genitive in Old English; it is uncertain from the available evidence whether there is continuity between them. Examples are: OX-EYE n., OX-GALL n., OXGANG n., OXHERD n., OX-HORN n., OXLIP n., OXTAIL n., OX-TONGUE n. Compounds of oxen, which may represent either a reflex of the Old English genitive or the plural, occur sporadically in Middle English (cf. OX-EYE n. 2b, OXHOUSE n.). Recent compounds with the plural oxen are included among examples of compounds with the singular (see senses 6, 7); some cases may be accounted for by the influence of a plural second element.]
Ox is the only word in general English use which retains the orig. plural ending -en (the reflex of Old English -an) of the weak declension. An older umlaut plural xen, exen occurs in Old English (Mercian and Northumbrian respectively), whence app. the Old and Middle English forms exen, exin, and Middle English exon. A new plural oxes, found from the 14th cent., has survived only in regional and non-standard use. The genitive singular oxes for oxan appears in Old English (Northumbrian) in Lindisf. Gospels (cf. quot. OE1 at sense 1a), and an -s genitive is normal from the Middle English period onwards.
A number of compounds of the base form of the word correspond to compounds of the genitive in Old English; it is uncertain from the available evidence whether there is continuity between them. Examples are: OX-EYE n., OX-GALL n., OXGANG n., OXHERD n., OX-HORN n., OXLIP n., OXTAIL n., OX-TONGUE n. Compounds of oxen, which may represent either a reflex of the Old English genitive or the plural, occur sporadically in Middle English (cf. OX-EYE n. 2b, OXHOUSE n.). Recent compounds with the plural oxen are included among examples of compounds with the singular (see senses 6, 7); some cases may be accounted for by the influence of a plural second element.]
