There are many different themes in The Odyssey, most of which are shared by other epics. One of the most notable would be the relationship between gods and mortals. The gods of Olympus make regular appearances throughout the poem, intervening at crucial moments in the action to make things happen. The most prominent Olympian in the Odyssey would have to be the goddess Athena, who fortifies Telemachus's courage as well as assists Odysseus in his quest to return to Ithaca.
Not all of the gods' interventions are helpful to mortals, however. The mighty Zeus, father of the gods, sends down a huge thunderbolt that destroys Odysseus's ship. This is payback for Odysseus's men killing and eating the Oxen of the Sun, which they were expressly warned by Circe not to do. Zeus carried out this terrible vengeance at the request of the sun-god, Hyperion, whose cattle were slaughtered and eaten by Odysseus's crew.
What this episode illustrates is that mortals must always know their place when it comes to the gods. The gods may sometime intervene to help them in their hour of need, but they will turn on them without a moment's hesitation if they should take any liberties.