Stanza 2 of Bronson's version 27 is

This little MacGroves a-being there,
All dressed in oil of green,
He looked at her, she looked at him,
The like was never seen.

(headnote ref - Sharp MSS, 3686/2742. Also in Sharp & Karpeles, 1932, I, p175(J). Sung by Mrs Delie Knuckles, Barbourville, Ky., May 16, 1917.)

So there it certainly is in at least one authenticated version ~~ not 'olive', not 'holy', not 'all in green', not a mondegreen or whatever: but right there in Sharp, who was always a very careful transcriber. So the OP's question surely stands.

This is the only variant I can find after a quick haul thru Child & Bronson which specifies a colour that Musgrave/Matty/MacGroves &c wore; as distinct from the ladies, whose colours are specified in many versions. But I have long sung this ballad with a stanza almost identical to that above, but with "Lincoln green"; which I think I learnt from Paul Carter's singing of a Canadian variant called "The Young Leboux", way back in the late 1950s. But that is longish since & I would not swear to my source. Still, whatever that source may have been, "Lincoln green" is what has been in my consciousness over the 55+ years I have known the ballad.

~M~