Wasn't there an episode where Porthos did something on a planet that upset the local so much that Archer had to apologise for it, the same one that gave us the famous Gazelle speech? If it was shown onscreen that could've been an oppurtunity to hear an "Oh boy" as we see Archer and co walking along, Porthos wanders off, does his thing, Archer goes to look for him and finds him doing something that he shouldn't be doing and that's when he says his oh boy line.
Another oppurtunity could've been on the Risa episode where instead of Trip and Malcolm being mugged by the gender changing thieves make it Archer and Trip and when the thieves reveal themselves Archer could give the oh boy line there.
Leland Orser (Loomis) got to say it quite loud in
Carpenter Street, which Scott winced at - possibly unscripted.
Isn't "Oh Boy" one of the many lines he says in all his movies? Every one I've seen him in he always seems to play the nervous type of guy.
