An air carrier crew departed HNL on the OPIHI TWO DEP and had a track deviation when they entered KOKO (KOKO HEAD VOR - CKH) instead of CHOKO intersection.

Narrative

We were cleared departing HNL via the OPIHI TWO DEP; climb and maintain 5000'; expect FL300 10 minutes after departure. Tower assigned a 155 degree heading after takeoff. I was the Relief Pilot in the primary observer seat; the First Officer was the pilot flying and Captain was the pilot not flying. The turn southeast (155 deg) was executed slightly late (~600') and we had been told to 'start our turn' by Tower. Shortly thereafter (~1300') we were told to switch to Departure (118.3). Upon switching and climbing to 5000'; we were told to climb to 10;000' and were cleared direct to CHOKO (part of our routing); though we had expected vectors to OPIHI (per the SID). After selecting 10;000' in the altitude window; the crew mistakenly heard 'Koko' instead of CHOKO; saw the CKH (Koko Head) VOR on the NAV display (couldn't quite understand why Departure wanted us to reverse course; but we didn't question or clarify it); the Captain typed CKH in the scratch pad; selected Direct Intercept to CKH; which I thought was the clearance. Shortly after or prior to rolling out; Departure told us to turn right to 245 deg -- 'NOW.' The controller reconfirmed that we were cleared to CHOKO. After selecting 245 deg and as the First Officer commenced the right turn; the CAPT scrolled down to CHOKO; and then selected the correct clearance fix. We then advised DEP that we had heard 'Koko;' apologized; and preceded on course to CHOKO. Aside from the proximity of Waikiki; there were no traffic conflicts. None of us questioned the course reversal; but clearly (now) it made no sense; especially in view of the HNL 10-4 Commercial Chart 1 page. ['Turn southward as soon as possible after takeoff. Remain at least one mile offshore Waikiki . . . .] Our altitude by the time we had re-established routing to CHOKO was in excess of 5000'. Clarification with ATC and definitive verification from the other crewmembers would have precluded this event. The fact; the First Officer was 'hand flying' the aircraft took him somewhat out of the loop; but the error was inexcusable. It should NOT have happened. Although I have not flown this segment; I should have studied the departure and surrounding NAVAIDs in more detail to avoid confusion with similar sounding points.