Omaha is a community card game played between 2 and 8 players. The most common number of players are 2 (heads-up) and 8 (full ring). The objective of the game is to make the best 5-card hand selecting 2 cards out of 4 private hole cards and 3 out of 5 community cards which are shared among all players.
Each player is initially deal 4 cards face down. Otherwise the gameplay is exacly the same as for Holdem.
The average winning hand in Omaha is generally a lot better than in Holdem given the additional possibilities having a total of 9 cards to make a hand from instead of just 7. Very often a player will be holding 'the nuts', ie the best possible hand given the communitity cards on the table.
It is very important not to misread an Omaha hand. For example, 4 of a suit on the board and the ace of the suit in a players hand does not constitute the nut flush as 2 cards must be used from a players hand. Also, 3 of a kind in your hand (or worse, quads) is a very poor starting hand multiway as it makes it more difficult for the hand to improve with the cards that are left in the deck. It is important to be aware of the concept of of dominating and dominated hands. A 4 card Omaha rundown hand maybe look promising but could be drawing thin against a slightly better hand that shares some of the same cards.
Omaha is played with a pot limit betting structure. Omaha is a game that sees a lot of action given the greater possibility of making a strong hand with having 4 cards to use instead of just 2 as with holdem. For this reason a no limit betting structure would generate too much action with players being priced in at all stages to continue in the hand. See the section on Betting Limits for more information