Hold up. Do you put stock in that theory, Dean?
More than I do 'Andraste was a mage,' actually (and slightly below 'she was a hedge mage without training).
I think it was TheDarkKnightReturns who sold me. Andraste's birth year is within an acceptable range for an OGB of Dumat, with enough time and access for the mother to get to a Ferelden fishing village before it's birth. The Sentinel armor set, or the armor of the Warden who slew Dumat, somehow ends up in Ferelden near a coastline (the birthplace of Andraste) rather than be reclaimed and stored as a Warden relic at Weistephat, which wouldn't make much sense if the First Warden died. To top it off, Flemeth and Morrigan had to have some reason to believe the OGB ritual would work.
It's highly circumstantial, but if it were true it could also explain some of Andraste's feats. If she were an untrained mage, as a hedge mage her magical powers would manifest in ways not necessarily under her control or at her intended direction. If she were an OGB hedge mage, her magical nature being released in untrained ways could manifest as miracles that no regular mage could mimic.
So, as a theory that works as well as any other...
The First Blight ends with an OGB ritual by whatever the equivalent of Morrigan at the time would be. The Sentinal Warden, the one to kill Dumat, is in on it. He (she?) fakes their own death and flees the battle with not!Morrigan and their armor and flee to the farthest reaches of the known world, as far away from the Imperium's watch as they can. They go to Ferelden, by boat, and end up at a coastal village when Andraste is born. The Sentinal Warden watches over for a time but loses the armor.
Andraste grows up not knowing she is the OGB. She is magical, but not trained as a mage, and so her magic manifests as a hedge mages does: suppressed, subtle, and not necessarily under her own control. She goes her life feeling something different, but never knowing what, and so her own magical influence that responds ever so slightly towards her is projected onto the idea of the Maker. Andraste fights, her powers assist her in ways that no conventional magic could, and not knowing to credit herself Andraste credits the Maker. History happens. Andraste's not-deliberate OGB magic starts a religion dedicated to monotheism that dominates the continent and defines the politics of the next millenia.
So, the next time someone asks 'what's the worst the Old God Baby could do?', ask them this: 'How do you feel about the Chantry?'
[But no, really, I think it's plausible enough to not reject.]