Chalice 2 against Oath seems really good. Maybe I'm crazy but turning a two card combo deck into a Show and Tell deck(if they even play it) is pretty good.
Metalworker seems really bad against Oath. Maybe I'm crazy but turning a two card combo deck into a one card combo deck seems like a weak plan.
tl:dr
-4 Metalworker
-2 Staff of Domination
-1 Wurmcoil Engine
+4 Grafdigger's Cage
+1 Duplicant
+1 Triskelion (can kill the Orchard Tokens + itself after you Waste/Quarter/Strip the Orchard)
+1 Ratchet Bomb (set to 2, kill Oath's)
If you want to force the Karakas in (it's terrible against Griselbrand) you can, but I am not sure what you want out. Maybe 1 Crucible and one Lodestone.
I agree with everything you said Tom, you are spot on with your board strategy.
Further, if Oath is really a big player in your metagame, then the best way to combat Oath is probably to play a different Workshop list. Personally, if I were you, I would play either Espresso, Martello, Marinara or Terra Nova, as no offense to you, but they are better suited to do well than your list. Also, Metalworkers are inherently weak against Oath, because as Tom said it turns their 2 card combo into a one card combo and unlike Kuldotha Forgemaster, does not provide an immediate answer to the Oath.