The template will depend on the type of oracle:
If it's an Oracle of Apollo, does this mean the prophecies only relate to apollonic themes? After all, if the visions come from Apollo then he may only provide glimpses of what he cares about (though presumably he cares for his oracle too).
Are the visions clear and understandable, or are they cryptic? Will the PC be aware of the prophecy, or is she just the mouthpiece and has no knowledge of what she's said.
Are visions always compels, or can the character invoke a vision as well? Can other people force the PC to have a vision? (Certain factions might want to kidnap the PC to use as their own personal crystal ball).
If they can, do they need some kind of ritual? Do they need to be under-the-influence of a particular substance? Or can they just meditate a while? (Praying to Apollo, tarot cards or palm reading, hallucinogenic substances etc).
Is there a practical side to the visions? For instance do they get flashes of insight mid conflict which allows them to say or do the right thing in a social situation, or be aware of an ambush before it happens, or duck at just the right time? Or is that just a case of invoking the aspect for re-rolls and so forth.
What is the relationship between the PC and Apollo? Does she worship him? Was she chosen by him? Or did it happen by accident i.e. wrong place wrong time, item of power etc. Is she happy to be an oracle or is this a curse?
As for rules, this could be treated like sponsored magic, casting divination rituals at evocation speed, debts owed to Apollo etc. Or it could be a supernatural power as DMW has suggested above.