Sorry, McGrath. I did include "at" in my initial list. It could be something like "at dinner."

But how could it be a conjunction like "or?"

The clue pretty clearly indicates we're looking for something a polar bear could BE. That means the 2-word phrase would have to serve as a noun (e.g., "an animal") or an adjective (e.g., "so stupid").   The 2-letter word has to be a preposition (e.g., "at") or an article ("an) or an adjective (e.g., "my") or an adverb (e.g., "so").

So that pretty much eliminates two-letter words like these (unless it's some weird pun):
be
do
go
hi
he
it
if
is
lo
me
or
us
we
ye