In hold ‘em, if you make a ten-high flush (using both cards in your hand), that's a very strong hand. In Omaha, a ten-high flush is not a bad hand to have, but you need to tread very carefully and will have to fold if it looks like someone really likes their hand. In Omaha, since you have four cards in your hand instead of two, strong hands are much more likely.
If you're facing a bet in hold ‘em and you're holding four cards to a ten-high flush, it's often correct to call. In Omaha, it's bankroll suicide. Hitting your hand and being forced to fold (or worse, losing your stack) is a disaster, and if a lot of money goes in, you can be pretty confident you're going to get shown an ace-high flush. In Omaha, you want your draws to be to the nuts (the top possible hand). If you're in a free roll, it's less important as you're 9-high flush will probably get paid off by a straight, two pair, or some other junk hand.
