What shaft? On the distributor? It only has one shaft and it isn't really attached to the engine, it's right on top. If oil is leaking down below, that is a separate issue.
There is one o ring on the outside of the distributor housing that you are describing.
There is also an o ring internally to the distributor that seals the shaft to the INSIDE of the distributor housing. This one is not serviceable.
The distributor is known to leak externally And occasionally internally that fills the inside of the cap with oil.
The reason the FSM doesn't show it us because it's a non-serviceable part. I'm sure someone here has seriously attempted a rebuild, but unsuccessfully or extremely costly.