Omaha has 2 different variations. The normal and high/low one. We will explain both starting with standard Omaha.
Omaha is played the same way as Texas Holdem so you can see those rules for all betting rounds. But Omaha has one exception. Players are dealt 4 cards and MUST use 2 cards. (So this means when you only have 1 Ace you can never make quads or when you only have 1 hearts, you can never make a flush of hearts)
Omaha High/Low is played the same as above but now you must make a low and a high hand. A low hand is normaly considered to be 8 or higher. One player can use 2 cards to create a high and 2 cards to create a low hand. So he can win both. But when someone has a low and another player has a high hand, both of them win half the pot. If no low hand exists the high hand receives the whole pot.
Most Omaha traffic is on Full Tilt Poker, wich also offers an amazing $600 bonus for new players.
Below you will see a omaha hand played: