[iggymcfly]

You must play a weird brand of Omaha. I don't think I've ever played with a single player that I could put squarely on top set after betting the pot on the flop. What does he do with middle set here? Check/call? Check/fold? Against a typical player, I'd put them on a range of two pair, a set, a wrap, occasionally an open-ender, and occasionally an overpair/air. Against that range, I'd repot with any set or any wrap.

Now I understand if you can narrow that range down somewhat against this player, maybe even to any set, but I don't see how you can say that he absolutely has top set here. However, if you can do that, then this is an easy call. You have an advantage over top set by calling with the wrap here since when he hits his good cards (board pairs), you'll be able to fold, but when you hit your good cards (straight cards), he'll still be calling you.

Now, the other thing mentioned (that he might check/fold top set on the turn when a straight card hits), is this really possible? What kind of ridiculous uber-nits are you playing with? Most of my opponents will never lay down any set here and no one check/folds top set. If your opponents really are as nitty as you describe, you need to be making a lot of light calls and stealing pots like crazy on every street.