B737-300 FLT CREW RPTS TURNING ONTO RWY 18 WITHOUT CLRNC AFTER LNDG RWY 14R AT OMA.

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I WAS THE PF. I HAD A STABILIZED APCH AND LNDG. I KNEW THEY WERE USING THE 14'S; BUT I OVERLOOKED THAT RWY 18 WAS ALSO BEING USED. I NEGLECTED TO BRIEF MY RWY EXIT POINT SPECIFICALLY; I JUST MENTIONED THAT WE WOULD EXIT TO THE R. UPON ROLLOUT; I SAW RWY 18 AND APPLIED THE BRAKES AS THE AUTOBRAKES WERE OFF WITH 9500 FT OF RWY AND GOOD VMC WX. I ASKED THE CAPT; AS I WAS SLOWING DOWN TO MAKE THE RWY INTXN; IT HE WANTED THE RWY; HE SAID 'SURE' AND WE XFERRED CTLS. UPON THE ENTRY ONTO RWY 18; WHICH WAS AN ACTIVE RWY; BUT NOT BEING USED BY ANY ACFT ON FINAL OR TKOF; TWR KINDLY REMINDED US THAT RWY 18 WAS AN ACTIVE RWY AND THAT WE NEEDED CLRNC TO ENTER IT. I USUALLY QUERY TWR IF RWY IS AVAILABLE FOR TAXI; BUT IT GOT ME TODAY. I DO FEEL THAT IF I WAS MORE INFORMATIVE ON MY APCH BRIEFING DECLARING A SPECIFIC TXWY TO TURN OFF WITH A POSSIBILITY OF A RWY EXIT; I FEEL THAT WE AS A CREW WOULD NOT HAVE COMMITTED THIS RWY INCURSION. ALSO; IF I READ THE ATIS MORE THOROUGHLY AND SAW THAT RWY 18 WAS AN ACTIVE RWY; I KNOW THAT I WOULD PROBABLY JUST ROLL THROUGH THE ACTIVE AND TAKE THE NEXT AVAILABLE TXWY. THERE WAS NO NEED TO TAKE RWY 18 FOR PERFORMANCE REASONS AS WE LANDED WELL WITHIN THE TOUCHDOWN ZONE. THE NEED WAS FELT TO BE SINCE WE WERE RUNNING APPROX 10 MINS LATE WITH A SHORT TURNAROUND THAT A SHORTER TAXI TO THE RAMP AND GATE WOULD BE GOOD. BUT I KNOW THAT THE ABOVE STATED REASON WOULD NEVER HAVE HAPPENED WITH A PROPER RWY EXIT BRIEF AND BETTER SITUATIONAL AWARENESS. SUPPLEMENTAL INFO FROM ACN 792264: AN IMMEDIATE EXIT WAS MADE ONTO THE PARALLEL TXWY.