Omaha Poker, or Omaha Hi, is a variation of Hold 'em played with four hole cards instead of two. At showdown, the best five-card hand using exactly two of your hole cards and three cards from the board wins. Unlike Hold 'em, you may not use more than three cards from the board. The betting and blinds are exactly the same as in Hold 'em.
At the beginning of each hand, each player is dealt four hole cards. There is a small blind, a big blind, and a round of betting.
In Pot Limit, a player can bet any amount from the minimum bet to the size of the pot. In a $5/$10 pot-limit game, the small blind is $5 and the big blind is $10. The first player can call the big blind (in this case, $10), or raise to any amount up to the size of the pot. A raise must either equal or exceed the previous bet or raise. In this case, a pot-sized raise would be $25 ($5 small blind + $10 big blind + $10 call), meaning that, in total, player three can bet up $35. Supposing player three makes a pot-sized raise, the total pot is now $50.
If the next player wants into the hand, they must call $35, which is the size of player three's bet. If they want to raise the maximum, they would bet $120, which equals the size of the pot ($50) plus a $70 raise ($35 call + $35 raise).
Once the first round of betting is complete, the dealer then puts out a three card flop, just as in Hold 'em. There is a round of betting. In the above example, the minimum bet would be $10 (the size of the big blind), and the maximum bet would be the size of the pot.
All raises in all remaining rounds behave as described above.
The dealer then puts out a fourth (or turn) card, followed by a round of betting.
The dealer then puts out a fifth (or river) card, followed by a round of betting.
The winner is the player who makes the best possible five card hand using exactly two cards from their hand and three cards from the board.
This is important. Unlike Hold 'em, in Omaha Hi Poker you cannot use just one card from your hand, nor can you play the board.
For example:
Your hand: Ah Kc Qc Jc The board: 3h 4h 5h 6h 7h
You DO NOT have an ace-high nut flush, nor can you simply play the board for a straight flush. Using two cards from your hand, your best hand is:
Ah Kc 7h 6h 5h
In other words, you should have folded.
Information from Full Tilt Poker