FLC DISTR AND LACK OF FAMILIARITY WITH ONS; ACFT WAS OFF COURSE.

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SCHEDULED FLT MIA-GCM; TRANSITIONS CUBAN AIRSPACE G448 AT FL337; CLRED DOWN TO FL290; THEN PLT'S DISCRETION TO FL190. HAD PREVIOUSLY BEEN CLRED TO FL370; REQUESTED FL330. AS WE PASSED WERE CLRED BACK TO FL330. I RESET ALT WARNING FROM FL370 TO FL330 (AUTOPLT ENGAGED WITH ALT SELECT) AS I WOUND ALT DOWN TO FL330; AUTOPLT CAPTURED; NOSE PITCHED OVER. FO DISCONNECTED ACFT TO SMOOTHLY TRANSITION ALT CAPTURE. AUTOPLT REENGAGED; BUT APPARENTLY ONS NOT RESELECTED AS NAV MODE. PASSING UCL SBOUND; NOTICED FULL DEFLECTION (EXCESS OF 7.5 NM) R OF COURSE. WE WERE PROBABLY 40-50 NM S OF UCL. IMMEDIATELY TURNED 40 DEGS L TO INTERCEPT COURSE SBOUND. FO NOTED THE ONS DEFLECTION ON #1 OMEGA; THEN NOTICED NO GREEN AUTOPLT ANNUNCIATOR LIGHT. BOTH ADF NEEDLES ON 230-UCL VOR TUNED TO GCM 115.6 BUT INBOUND COURSE (179 DEGS) HAD NOT BEEN SELECTED YET. DURING THIS TIME; THE FLT ATTENDANT HAD COME INTO COCKPIT TO DISCUSS A SEATING PROB. I HAD BEEN DISTRACTED. FO SAID HE'D BEEN MILDLY DISTRACTED. IN THE MIDST OF THIS; THE FO THEN NOTED THE COURSE ERROR. CAPT (ME) HAD USED ONS REGULARLY OVER LAST 3 YRS. FO HAD NEVER USED IT OTHER THAN OCCASIONALLY OVERFLYING DOMESTIC AIRWAYS. I HAD NEVER BEEN TO GCM. FO HAD BEEN THERE ONLY ONCE; SO NUMEROUS TIMES. ONCE REESTABLISHED ON COURSE CTRLINE; ASKED HAVANA CTL (133.7) IF HE SHOWED US ON COURSE. HIS RESPONSE WAS NOT QUITE CLR. HE DID NOT ADVISE US IF HE NOTED ANY NAV DEV. I HAD BEEN PREOCCUPIED WITH 'PROPERLY GETTING IT ALL DONE.' I OVERLOOKED THE BASICS; LOST SIGHT OF THE FOREST BECAUSE I WAS TOO WORRIED ABOUT THE TREES.