It did, but whether the Dominion had authentic knowledge of those, we don't know for sure.
Sisko did in that episode bring aboard members of the Cardassian Detapa Council, and Klingons were convinced those were Changeling infiltrators. If they were, they would have been able to monitor the battle in which the upgraded weapons were used - sneaking out of their designated protective rooms would have been simple enough.
Whether the shields were upgraded at that time already or not, we don't know for sure...
I doubt O'Brien had much to do with the shield upgrade, though. Why would the janitor of a wayside space station decipher the mystery of protecting against phased polaron beams, and not some research team back on Earth or Vulcan Arcturus?
Of course, we have to remember that one of the station's runabouts took ppb hits way back in "The Die is Cast" already, and survived. But those might have been glancing hits, or deliberately pulled punches to merely stop rather than kill Odo. Hits taken by the Defiant might not count, as she has this ablative armor thing in addition to the possibly still useless standard shields.
Timo Saloniemi