This work presents two quite fascinating and well thought out 'mysteries' which Oracles might choose. These are Fathoms and Lunar.
Firstly, Fathoms is a distinctly wet option! It involves water, and grants the Oracle the ability to swim as well as adding Knowledge (nature) and Survival to his class skills list. As he rises in level (or sinks to the depths?) he also develops other watery powers such as the ability to breathe underwater and descend to extreme depths without problems. As water, especially deep down, tends to be cold the Oracle also gains benefits in casting cold-related spells. All-in-all it is an interesting package especially if your adventures are at sea or at least on a shore...
The other, Lunar, involves the moon, of course (assuming your campaign world has any!). The abilities conferred on Oracles that study this mystery are many and various ranging from skill at Acrobatics, good perception, a talent for both diplomacy and bluffing and even the ability to frolic the night away! Low light presents no obstacles, and other gifts are based on the 'inconstant' nature of your average moon... the waxing and waning in the sky. Again, it's quite fascinating and appropriate to any Oracle wherever he might adventure - after all, the moon shines down on everywhere too.
They are both thorough and consistent mysteries, worthy of consideration as alternatives to those in the core books.