B737 FLT CREW HAS AN UNSTABILIZED APCH INTO ORD.

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ARRIVING INTO ORD ON THE OXI 1 ARR; WE WERE ASSIGNED RWY 22R BY APCH; AND VECTORED ACCORDINGLY FOR A L DOWNWIND. I (PF) IMMEDIATELY BEGAN SLOWING THE ACFT BY EXTENDING THE GEAR AND FLAPS TO 30 DEGS. SHORTLY AFTER WE BEGAN SLOWING; WE WERE VECTORED BY APCH TOWARDS THE RWY 22R FAF AND GIVEN A XING ALT; WHICH WAS SLIGHTLY ABOVE NORMAL GS INTERCEPT ALT THERE. REFING THE EHSI 'GREEN ARC;' IT WAS CLR THAT WE WOULD ARRIVE OVER THE FAF SLIGHTLY HIGH; SO WE EXTENDED FLAPS TO 40 DEGS TO INCREASE OUR RATE OF DSCNT. WE WERE THEN CLRED FOR A VISUAL APCH TO RWY 22R. AT THE ESTABLISHED CONFIGN AND RATE OF DSCNT; WE ANTICIPATED REACHING THE GS AND TARGET SPD INSIDE THE FAF; BUT BEFORE 1500 FT AGL. AT APPROX THE FAF; THE RWY 22R GS 'CAME ALIVE' AND WE WERE WELL ON TRACK TO MEET ESTABLISHED STABILIZED APCH CRITERIA EARLY. AS WE CONTINUED TO CAPTURE THE GS; THE GS INDICATION ON THE EHSI QUICKLY PASSED THROUGH CTR; AND WENT TO FULL SCALE TOP (BELOW GS). DUE TO THE CONFLICTING INFO BTWN THE ILS GS AND OUR EHSI GREEN ARC; I STOPPED MY DSCNT TO RE-EVALUATE OUR SITUATIONAL AWARENESS. DURING THIS PAUSE IN DSCNT; WE ONCE AGAIN WENT HIGH ON THE ACTUAL (NOT INDICATED) GS; AND THEN ALL ILS INDICATIONS (GS AND CDI) COMPLETELY DISAPPEARED ON OUR EHSI; WE LATER ASSUMED BECAUSE OF INTERFERENCE FROM A PRECEDING B777 CLRING THE RWY. ONCE WE CONFIRMED OUR POS RELATIVE TO THE TERRAIN; WE ATTEMPTED TO RECOVER THE APCH. NOW WITH THE RWY AND VASI CLRLY IN SIGHT; IT APPEARED THAT RE-ESTABLISHING THE GS WOULD BE POSSIBLE BEFORE THE 500 FT LIMIT. AROUND 1000 FT; THE GS ONCE AGAIN CAME ALIVE; AND WE FELT CONFIDENT WE COULD MEET STABILIZED APCH CRITERIA. AT 500 FT AGL; WE WERE STILL SLIGHTLY OUTSIDE STABILIZED APCH PARAMETERS; BUT THE ACFT TREND SHOWED US REACHING IT MOMENTARILY. I ASKED THE CAPT 'SHALL I CONTINUE?' AND THE RESPONSE WAS; 'YES; CONTINUE.' I BELIEVE WE MET ALL STABILIZED APCH CRITERIA AROUND 400 FT AGL. A NORMAL FLAPS 40 DEG LNDG RESULTED; FOLLOWED BY A NORMAL TAXI TO THE GATE. AFTER COMPLETING OUR PARKING CHKLIST; THE CAPT AND I RE-EVALUATED OUR DECISION TO CONTINUE THE APCH. WE BOTH AGREED THAT EXECUTING A MISSED APCH/GAR WOULD HAVE BEEN BETTER; AND THE CORRECT OPTION. CONTRIBUTING FACTORS TO THIS EVENT WERE: BEING 'SET UP' BY ORD APCH TO GET HIGH; CLRNC FOR A VISUAL APCH IN MVFR CONDITIONS; POSSIBLE ILS INTERFERENCE FROM A PRECEDING ACFT; CONFLICTING APCH GUIDANCE INFO RESULTING IN RE-EVALUATING OUR SITUATIONAL AWARENESS; AND OUR ATTEMPT TO RECOVER A POORLY SET UP APCH. FACTORS WHICH LED US TO CONTINUE THE APCH WERE: A VISUAL CONFIRMATION OF TERRAIN CLRNC AND THE RWY; FULL ACFT CONFIGN ALLOWING A GOOD PREDICTION OF APCH RESULTS; AND THE JOINT BELIEF THAT WE WOULD MEET STABILIZED APCH LIMITS. WHILE NO PART OF THE FINAL APCH SEGMENT FELT 'UNCOMFORTABLE' TO EITHER PLT; THE FACT OF THE MATTER WAS THAT WE WERE NOT ABLE TO MEET STABILIZED APCH CRITERIA BY 500 FT AGL.