Is it correct to write 'An oxen'.

Yes, it's right.

About as right as saying "a oxen."

The last response by Misha was more sarcastic than enlightening.

Ox is singular, oxen is plural, so we can't say 'an oxen', just like we can't say 'an children'.

Why then, pray tell, did Ayatulla's answer go unchallenged for over a week? Was it beneath someone's dignity to offer a correction?

Let me try to grasp the logic of your last response: because an error went "unchallenged" for a week, that justifies your giving a sarcastic response to the poster?

Hmm…nasty, nasty...

You should have seen some of my other responses; you would have blanched. Have you set yourself up as the arbiter of sarcasm? Did that drive your answer to the original question?

"Have you set yourself up as the arbiter of sarcasm? "
"arbiter"? No.

"Did that drive your answer to the original question?"
Most definitely. If all you were offering in response was sarcasm, it is only being human to intervene to be more helpful.
And I won't hesitate to do so in the future, if necessary.

"You should have seen some of my other responses."
Sounds to me as if you feel you can throw your flyweight around in this forum - and feeling smug in the process!

And I won't hesitate to do so in the future, if necessary.
I would hope you would as it helps to have an alternate view, albeit at times incorrect.

Sounds to me as if you feel you can throw your flyweight around in this forum - and feeling smug in the process!
Of course. It's part of character building. And you know how many characters there are on this site!