I haven't been to OIB in several years, but my riding has always been confined to a loop or two between Calabash, Sunset and OIB. If you are on OIB come off the island go straight at the light. That would put the airport on your right. In a mile and a half or so you should come to Old Georgetown Rd. Take it to the left and it will take you into Calabash. Take a left on Beach Drive SW until it ends, then take a left onto Shoreline Drive W all the way to the Sunset bridge. You can go right onto Sunset and ride up and down the island or turn left (at the liquor store I think. There also used to be a fish market called Bill's on the right that was the best place to buy fresh seafood.) and ride toward OIB on Sunset NW. After a few miles you will return to the light near the airport where you can make a right turn onto OIB.
If I remember correctly, this loop is probably 18 miles at best, but if you add the loop onto Sunset Beach and do OIB from top to bottom, you might get over 30.
Hopefully someone else will chime in because my experience is from several years ago, traffic patterns change and it might not be healthy to ride that route any longer.