The Big Difference from Hold'em
In Hold\'em you get 2 cards and can use both, one, or zero. In Omaha — 4 cards, but you must use exactly 2. This changes everything.
Example: you hold A♠K♠Q♦J♦ on a board with 4 spades. You do NOT have a flush — you only have 2 spades in hand. Omaha requires 2+3. Many new players fall for this rule.