One begins with a vowel and should therefore have an and not a in front of it. Why is it, then, that ‘such a one’ is what is actually said?
It appears to have been the case when the King James Bible was translated in 1611:
1 Corinthians 5:5
To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
Moreover, the google yield of
"such an one" loeb has several instances from the 20th c.
One hypothesis would be that earlier orthography was considered to stand above phonology. Is that the case?