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].
Answer from TimR on Stack ExchangeOld 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].
I don't know of any satisfying reason for it.
Note that when the OED says "Old English– oxen (rare)", it means that the specific spelling O-X-E-N was rare in Old English. It doesn't say that oxan was rare, and from the point of view of later development, the difference between Old English oxan and oxen is irrelevant: it was normal for Old English "a" in unstressed syllables to be weakened to schwa, which in Middle English came to be spelled "e". Compare the development of the Old English infinitive ending -an to -e in the case of words like drīfan > drive.
There is a general principle that irregular forms persist longer in frequently used words, but I'm not sure how much it can do to explain the use of the form oxen. I don't think we talk about oxen as much as we used to.
It seems conceivable that the fact that the singular already ends in an /s/ sound made it a bit harder for the sibilant plural to become established, but I'm not really sure if that played an important role: obviously there are multiple other words ending in -x that do form their plurals in -xes, such as foxes, boxes, axes.