Looking at the WR dictionary, I think I use the expression wrongly; it has a similar definition to yours.I have not heard po-faced for a long time. My father used it when he thought I was making a disagreeable expression. Pole-faced, in my experience, means no expression, but forcing someone to listen to opera could make them po-faced!
http://www.worldwidewords.org/qa/qa-pof1.htmIt’s usually supposed to derive from the slang term po for a chamber pot (it rhymes with no), first recorded in the 1880s. But the abbreviation is more likely to be from the French pot de chambre than from the English term. It was probably influenced by the interjection pooh for something that’s distasteful, which is of course related to the verb to pooh-pooh meaning to express contempt or disdain for something; both are conventional reformulations of the noise one makes with the lips when appearing to blow away something unpleasant.
I'm not sure, sorry. I found it quoted at the page to which I linked the word "po-faced"."Passive or expressionless" isn't really an adequate definition for its normal use, I don't think. Is that an old or secondary OED definition?