Well, technically, it's driven off the cam...
Ya, that O-ring can leak, mine was when I got my truck. My technique is to put the engine at TDC (use the timing mark on the crank pulley), then take the cap off (good time to get a new one). Leave the rotor on, put a piece of masking tape on the surface where the cap seats, make a mark with a Sharpie where the rotor is pointing, both before and just after you pull it out. Also make a mark where the dist holddown bolt is, so you don't mess up the timing and get it back exactly as it was.
