In Omaha each player receives four hole cards instead of two. The other major difference between Omaha and Holdem is that you must use exactly two cards from your hand and three cards from the board.
In Texas Holdem you can use any five-card combination from your hand and the board, but in Omaha you have to use two cards from your hand and three from the board. This makes the play much different. In Texas Holdem you receive two down cards while in Omaha you receive four.
It looks like you have more possible combinations to play but you have to remember that you only have six possible combinations using the two cards. The other factor to keep in mind is that if each player at a full table has 6 different combinations to choose from that means there are 60 possible combinations at the table.
This means you should be selective with your starting hands when playing Omaha, but many people are not. In Hold’em Aces or Kings are great hands; in Omaha they are not. Omaha is really more of a game that features straights, flushes and full houses.
Another thing you really want to keep in mind that is much different than Hold’em is that a player will end up with “The Nuts” far more often in Omaha. In Hold’em it is really rare that a player will end up with the best possible hand. In Omaha it happens all the time.
If you have a good hand, but not the nuts, you are in real trouble in Omaha poker. Drawing hands are everything in Omaha. When playing Omaha usually you will have to show down your hand to win. Another big difference between Omaha and Texas Hold’em is that drawing hands are going to hit more often than you think.
Omaha is far more a game of cards than Hold’em. In Texas Hold’em you can win with top pair, trips, or even high card. Pairs and sets just don't win that often in Omaha because someone on a draw is likely to win. Texas Hold’em and Omaha are not the same game.
Your play must be different, your strategies better suited to the game you are playing. Omaha is much more a game that requires patience and discipline. You must be aware of the odds a great deal when playing Omaha. While your opening hand may not always look very good, it may have great drawing power.