Old English oxan, plural of oxa, was very common, appearing in the psalter, the bible, and laws, among other places, although the spelling oxen is attested in only one place, in a document relating to Bury St Edmund's possessions, rents, and grants.

The genitive plural form of oxa, often with a syncopated vowel, was ox(e)na. That genitive form is attested in place names (cf. oxenaford, Middle English Oxenford, ModE Oxford, oxeneham ModE Oxnam, oxenesetene ModE Oxsettle Bottom, oxnaleage ModE Oxleigh and Oxley , a woodland clearing or a natural glade, meadow, lea, a plant name oxna-lib glossing Latin oleotropius ModE oxlip, ox-heal), as a unit of measure of land (oxnagang, ModE ox-gang, one-eighth of a "hide"), and in genitive (oxna-paeþ ModE oxens' path) and partitive genitive constructions (ic bohte fif getymu oxena, ic bohte an getyme oxena, ModE I bought five teams of oxen, I bought one team of oxen) and those uses appear not only in texts dealing with quotidien farming and mercantile situations but notably in passages from the Bible, which would frequently have been heard by audiences from all social and economic classes.

The plural appears as oxen and is very well-attested in Middle English in a wide range of texts. It appears in various spellings, including oksen, exen, oxon, oxen, oxsen, oxsin, ocsen (see the MED). Its appearance in proverbial contexts (Moche uolk of religion zetteþ þe zuolȝ be-uore the oksen. Many people of religion set the plow before the oxen) and laws is very strong evidence that it was widely used.

P.S. I have a copy of the Old English corpus and found these attestations by searching it. (I studied Old English and Middle English as an undergraduate and graduate student, back before the days of personal computers, but they haven't changed much in the interim.)

P.P.S. I stumbled upon a book, Working Oxen by Martin Watts. 1999. "... a survey of their use in Britain, their impact upon the countryside, and the relics that can still be found: yokes, bows, shoes, housing and place-names. Martin Watts is curator of the Ryedale Folk Museum in North Yorkshire." [Google Books description] and "Oxen were one of the most important sources of motive power in the British countryside... The working ox has left a lasting mark on the language, landscape and culture... Historians rarely mention or study them. It is as if a history of twentieth century were to ignore the impact of the tractor and the lorry. The purpose of this book is to redress that balance." [from the blurb on Amazon].

The word ox comes from the Old English oxa. In Old English, as in Indo-European languages in general (historically and even today), the number of a noun (singular or plural) and its function in a sentence—whether it was the subject, direct object, indirect object, or had some other relation to a verb or another noun—was largely (not solely) governed by sets of endings tacked onto it, or changes made to the vowels in it. These sets of endings or changes were called declensions, and each type of relationship associated with an ending is called a case.

There were a number of declensions in Old English; the two most prominent were the weak declension, containing the weak nouns, and the strong declension, containing the strong nouns. Old English oxa was a weak noun. The forms that we have of its descendant today are derived from the nominative case endings; these are the forms that would indicate that a noun is the subject of a sentence, or the forms that would be used when writing a list of nouns.

Since oxa was a weak noun, its plural form (the nominative plural form) was oxan. Over the course of centuries, the a "weakened" to an e, giving us oxen.

Fox, on the other hand, comes from the Old English fox, which was a strong noun; its Old English plural was foxas, whence we get foxes.

The source I used to confirm the declension of fox has an entry for the Old English box; however, it has no declension information. Using this translator, however, it appears that the nominative plural was boxas, giving us boxes.

Although Modern English has largely dropped the declensional suffixes we got from Old English, we occasionally see them peeking through, as we do here.