ONT ATCT LCL CTLR CLRS A B737 FLT CREW FOR TKOF WITH TFC ON FINAL FOR THE SAME PARALLEL RWY.

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AS WE APCHED RWY 26R IN ONT FOR TKOF; WE CONTACTED TWR AND WERE TOLD TO HOLD SHORT FOR IFR RELEASE. WE NOTICED AN ACFT 3 MI OUT; AND IT APPEARED TO BE ON FINAL FOR RWY 26L. AFTER ABOUT 30 SECONDS; TWR CLRED US FOR TKOF ON RWY 26R. I VERBALIZED 'FINAL IS CLR; THAT GUY IS LNDG ON THE L;' AND I TAXIED ONTO THE RWY FROM TXWY W (THE END) AND EASED THE PWR UP TO ABOUT 50%. ANOTHER CARRIER; THE TFC ON FINAL; CALLED 'TWR; WHAT'S COMPANY DOING?' I KNEW NOW HE WAS TRYING TO LAND ON RWY 26R; SO I KEPT MOVING AND CLRED THE RWY IMMEDIATELY AT TXWY V (500 FT UP). BEFORE TWR COULD RESPOND; THE OTHER CARRIER CALLED 'CARRIER IS GOING AROUND.' AS I CLRED THE RWY; I ASKED 'TWR; WHICH RWY WAS CARRIER CLRED TO LAND ON?' THEIR RESPONSE WAS '26R -- MY MISTAKE -- SORRY.' WE WERE CLRED FOR TKOF AGAIN AND DID SO UNEVENTFULLY. I CALLED THE TWR AFTER LNDG IN OAK AND DISCUSSED THE INCIDENT. SEVERAL FACTORS LED TO THIS INCIDENT: 1) WE WERE STILL ON GND FREQ WHEN TWR CLRED THE OTHER CARRIER TO LAND ON RWY 26R; SO WE NEVER HEARD IT. 2) WE WERE TOLD TO HOLD SHORT FOR IFR RELEASE; NOT LNDG TFC; LEADING TO THE PERCEPTION THE LNDG TFC WAS CLRED FOR RWY 26L. 3) THE COMBINATION OF PARALLEL RWY CLOSE PROX AND A L XWIND MADE IT LOOK LIKE THE TFC WAS HEADED FOR RWY 26L. 4) TWR FAILED TO KEEP THEIR RWY OPS STRAIGHT.