A lot of hopin' and prayin' and cussin'
The problem is caused by forcing 2 similar metals together and they bond or weld together.
Crush that o-ring until the plug seats on the cover and this is what you get.
Hit the plug with a hammer, tap it around the edge. You're not trying to break it, just upset the bond a bit. Squirt it with pen oil.
Then what you need is some stiff metal that fits the slot tightly, a used front brake pad fits the bill nicely. You can even reshape it a bit and it makes a nice wrench. Gently tap the metal into the slot till you feel it bottom. Get a big cresant wrench (remember you're not askin') place it on the pad and in contact with the cover, adjust it tightly and turn it as best you can w/out it camming out. If it didn't come out, go ask a sailor for some new words.

BTW, replace that dam o-ring when the plug seats or it'll happen again.