B757 FO RPTS BEING DISTRACTED BY RWY CHANGES DURING THE ARR TO ORD AND OMITTING THE PA TO THE CABIN TO PREPARE FOR LNDG. FLT ATTENDANT RPTS THAT A PAX WAS IN THE LAV FOR THE LNDG.

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THE 'APPROACH DESCENT CHECKLIST' WAS COMPLETE AS WE APCHED FL180. AT FL180 THE CAPT INSTRUCTED ME TO TURN ON THE SEATBELT SIGN AND I FOLLOWED THAT WITH MY STANDARD SEATBELT SIGN ANNOUNCEMENT. WE WERE ON THE BDF ARR BRIEFED AND EXPECTING RWY 27L AT ORD. WE CHECKED IN WITH CHICAGO APCH AND THEY TOLD US TO STAND BY FOR A RWY ASSIGNMENT. AFTER SEVERAL MINUTES WE WERE ASSIGNED RWY 22R. THE CAPT INSTRUCTED ME TO INSTALL THE RWY 22R APCH AND SET UP THE NAVAIDS. AFTER BRIEFING THE APCH; THE CTLR CAME BACK WITH A NEW RWY ASSIGNMENT TO RWY 27R. ONCE AGAIN I INSTALLED THE NEW APCH AND THE CAPT BRIEFED THE PERTINENT CHANGES. THE REST OF THE APCH WAS UNEVENTFUL UNTIL AT 7 NM FINAL THE CAPT CALLED FOR GEAR DOWN AND FOR THE 'FINAL CHECKLIST'. I PUT THE GEAR DOWN AND REACHED FOR THE CHECKLIST AND BEGAN TO ACCOMPLISH THE CHECKLIST WHEN I REALIZED I HAD NOT GIVEN THE 'PREPARE FOR LANDING' ANNOUNCEMENT TO THE FLT ATTENDANTS. I SELECTED THE PA TO MAKE THE ANNOUNCEMENT AND THE FLT ATTENDANTS WERE PERFORMING THEIR PREPARE FOR LANDING PA ANNOUNCEMENT. UPON HEARING THIS I MADE THE DECISION TO OMIT THE CALL ON THE PA BECAUSE THEY WERE ALREADY PERFORMING THEIR FINAL CHECKS. I FELT I WOULD DELAY THEIR PREPARATIONS IF I INTERRUPTED THEIR ANNOUNCEMENT BY BROADCASTING OVER THEM. I THEN COMPLETED THE REST OF THE 'FINAL CHECKLIST.' THE REST OF THE APCH AND LNDG WERE UNEVENTFUL. AFTER THE PAX HAD DEPLANED THE FLT ATTENDANT ADVISED THE CAPT THAT THERE WAS A PAX IN THE AFT LAV DURING THE LNDG. WHAT DID I LEARN: THE MULTIPLE RWY ASSIGNMENTS GOT ME OUT OF MY NORMAL RHYTHM OF WHEN I COMPLETE THE 'PREPARE FOR LANDING' PA. I USUALLY ACCOMPLISH THIS BETWEEN 7000 FT AND 5000 FT. I ALSO SHOULD HAVE MADE THE PREPARE FOR LNDG PA OVERTOP OF THE FLT ATTENDANTS COMPLETING THEIR FINAL PA ANNOUNCEMENT. I FELT RUSHED AS WE APCHED THE FINAL APCH FIX AND FELT IT WAS MORE IMPORTANT TO HELP THE CAPT WITH THE LNDG. I MADE A CHOICE BASED THE BASIC PRIORITIES: AVIATE; NAVIGATE; COMMUNICATE.