Abdiel has a point about the amulet.
Less savvy on 3.5e Wizardry though.
Wizards have to pre-prepare their slots every day. So each slot Oracle has is essentially under a "Dibs" system that can only be truncated by counter-spells or if she has some kind of spontaneous ability, which would usually be very limited to certain types of spells for whatever class or prestige class grants it.
So she doesn't have a "freed slot"; if she was offering that spell, that's because it's prepped and is useless unless used. The question is only when in a day (night) to use it.
Spontaneous Arcane casters in this setting (often Cha casters, with the exception of Bards) tend to be unhinged maniacs who don't properly control their magic through mage-like rigor. So, your Sorcerers, your Dread Necromancers, and, arguably, your Warlocks.