Can the oracle truly use his ability at any time? I'm thinking specifically an enemy is revealed, and before the player decides whether to defend, the oracle peeks into the future to decide if that's a good idea. I assume yes, but in many games, you can't interrupt an action to do another action like that...
Another case... hornet is drawn, oracle is at full health. Can he purposely lose a point to peek a the deck, and thereby avoid hornet special attack?
Yes, anytime. Even in the Hornet scenario.