I'll phrase this as an example. If an Oracle of Battle wants to cast Wall of Fire (from his mystery), what is the DC for the spell? The spell is a level 4 sor/wiz spell but a level 5 druid spell. Since it's not an oracle (or cleric) spell, which is the level to count for DC for the oracle?
It's entirely possible, and probable, and supported by precedent, that each of the nine mystery spells corresponds to the spell level that you can cast as an oracle of that level, which would make wall of fire a 4th-level spell for an Oracle of Battle 8.