Make sure it's the red heat-resistant o-ring. Mikes has them i think. I did mine in an emergency on the road, surprising amount of oil coming from that. first used a normal o-ring from o-riliey's, then when circumstances permitted replaced it with proper red one.
All from the outside. And yes, next time I had the cover off after that, I definitely checked to see if it (the washer) was there, and it was