[BruceZ]

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I haven't looked hard at the other numbers, but my guess is that they're at least slightly off. I believe you overlooked the Inclusion/Exclusion principle (sticky, for the love of GOD, lol). So you're double/triple counting times when more than one person has the hand in question.

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yeah I know about that but I knowin hold'em it's fairly clsoe to just any one person ahving it. but that's with something like what are the chances that someone has AA if you have KK. the chances of 2 people having AA are almost 0 so it's not worth considering

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That's irrelevant for your calculation. The fact that the probability of 2 people having A2xx might be small would mean that you could assume mutual exclusivity and just multiply by the number of opponents, which is the same as using just 1 term in inclusion-exclusion. You are using the approximation of independence which requires a different assumption, namely that the events of each player having A2xx are independent of each other, meaning that one player's results do not affect the probabilities for other players, so you can multiply the probabilities by raising to the 10th power. I don't think either one of these are very good approximations for omaha, and they appear to give answers that differ by over 10%.

See aloiz's post, and my response to his post, for the exact answers by the inclusion-exclusion computation. These numbers are higher than yours as predicted, and one is almost 12% higher. It looks like the inclusion-exclusion principle has really found its use in Omaha.