Gem use, master enslave, high astral and oracle.

[Peter Ebbesen]

As you describe it, there's nothing mysterious unless I misunderstand you.

An S10 Oracle with a RoS equipped is an S11 caster and can thus use 11 gems. This is what happens in your first example.

Strip it of the RoS and it is an S10 caster and can use 10 gems. This is what happens in your second example.

[WraithLord]

This is the kind of info that belongs in the wiki

[Jarkko]

Originally Posted by WraithLord

This is the kind of info that belongs in the wiki

This is the kind of info that is explicitly stated in the rulebook, and is also correctly stated in the rulebook Read rulebook page 89 "A mage can only use as many gems as his current skill level in that path".