Just wanted to add to this sticky a bit of info that might help some of you.
The leak that happens at the VCT solenoid usually isn't a problem with the o-ring; it's the way the solenoid is seated. Because there is only one offset screw holding it on, it is easy to think it is fully mounted when it is actually slightly askew, leaving room for oil to leak out. Make sure the solenoid is flush around the diaphragm housing, and then tighten the screw. Make sure the solenoid remains flush after the screw is tightened, too... overtorqueing may cause the solenoid to pop up at the opposite side.