plural of ox, it is the only true continuous survival in Modern English of the Old English weak plural (see -en (1)). OED reports oxes occurs 14c.-16c., "but has not survived."
Entries linking to oxen
ox n.
"the domestic Bos taurus" (commonly meaning the castrated males, used to pull loads or for food), Middle English oxe, from Old English oxa "ox" (plural oxan), from Proto-Germanic *ukhson (source also of Old Norse oxi, Old Frisian oxa, Middle Dutch osse, Old Saxon, Old High German ohso, German Ochse, Gothic auhsa), from PIE *uks-en- "male animal," (source also of Welsh ych "ox," Middle Irish oss "stag," Sanskrit uksa, Avestan uxshan- "ox, bull"), said to be from root *uks- "to sprinkle," related to *ugw- "wet, moist." The animal word, then, is literally "besprinkler."
Also used from late Old English of the wild, undomesticated bovines. The black ox "misfortune, adversity, old age," etc., is by 1540s.
-en 1
word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.