Hmm. Under normal circumstances, I would say that that would probably be a little too overpowered to allow to see play... However, this isn't a normal game. Phantom's already said to not worry about whether or not something is overpowered, so I say go for it. ^^
In fact, in that case, you could probably just use your innate casting for Cleric spellcasting anyway, and take an arcane class for your other side. You could take levels as a Wizard and use the Spontaneous Divination ACF from Complete Champion to get access to any divination spell in the game through your Wizard spell slots, for example, which would be great for an oracle like Arachne. Plus, you'd still be able to qualify for Divine Oracle like that, since five levels of Wizard could easily satisfy the necessary skill requirements.
And Divine Oracle can be used to progress Wizard casting, too, so you won't lose out on spell levels on either side.
And since the fifth level of Divine Oracle doesn't have much in the way of class features, you could take some other arcane casting prestige class for that last level to get some other nice abilities, too.
Edit: For example, Arcane Devotee from the Player's Guide to Faerun would both fit your character concept and give you a really nice ability, and it would be very easy to work into your build.