The hand rankings from Omaha are exactly the same as those for Hold'em. The main purpose of the game is to make a better five-card hand than your opponents. In Omaha you start with more cards in your hand than you do in Hold'em, so in general you require a better final hand to win at Omaha. However, the hand rankings are exactly the same, from royal flush all the way down to no pair.
For more information on hand rankings, please refer to the following sections: