Omaha is a variation of poker similar to Texas Hold'em but with following differences: each player is dealt four hole cards instead of two and to make their five-card hand, they must use exactly two of their hole cards and exactly three of the five community cards. So for each hand dealt, a player has 60 possible ways to make a five-card hand in Omaha compared to only 21 in Hold'em. All other aspects of Omaha are identical to Hold'em.
Omaha-5 is very similar to regular Omaha except each player gets five hole cards instead of four. They still must use exactly two of their hole cards and exactly three of the five community cards to make a hand.
Omaha Hi-Lo (also referred to as "Omaha 8 or Better") is a variation of Omaha where the pot is split by the players with the best high hand and the best low hand. The same player can win both halves of the pot. The high hand is determined in the same way as regular Omaha described above. The low hand is determined by the player who has five unpaired cards of the lowest rank, using any of his two hole cards and any three community cards. These do not have to be the same five cards used to make the high hand. Aces can count as low cards and none of the fives cards can be higher than an 8. Straights and flushes are not a factor in calculating the low hand so the best low hand is 5432A while the worst is 87654. Any combination containing a pair or a 9, 10, J, Q, or K is not qualified for a low hand so not every hand will have a qualified low hand. No player can make a low hand if there are not at least three community cards of different ranks 8 or lower. And no individual player can make a low hand if they do not have at least two hole cards of different ranks of 8 or lower. If there are no qualified low hands at showdown, the player with the best high hand wins the whole pot. If the pot contains an odd number of chips and there is at least one qualifed low hand, the extra chip is awarded to the high hand. Both the high and low halves of the pot can be split by multiple players in the case of ties.
Omaha-5 Hi-Lo is very similar to regular Omaha Hi-Lo except each player gets five hole cards instead of four. Omaha-5 Hi-Lo played under Pot Limit betting rules is often referred to as "Big-O".