Counting Outs in Omaha looks similar to Holdem but has some important, but subtle differences. Those differences stem mainly from the fact that Omaha 8/b is a split pot game.
Quick example: Your opponent bets into you on the turn. You have no chance for high, but can win low if you catch your card. After your opponent bets there are 6 big bets in the pot, and you must call 1 big bet to see the river - what are your pot odds? Answer quickly, then read on.
If you said "3 to 1 pot odds, 'cause the pot is 6 big bets and I can win half of that which is 3" - then you're wrong. That's a hold'em answer, and you're playing a split pot game now. Confused? Let's take a closer look at that example. You call the bet so there are now 7 big bets in the pot. The river comes, you hit your low, and you both check down. So you win half the pot, or 3.5 big bets. But you put in 1 big bet yourself, so your actual profit is 3.5 - 1 or 2.5 big bets. So when you called to hit your hand, you were actually risking 1 big bet for the chance to win 2.5 big bets. Your pot odds were 2.5 to 1 instead of 3:1. Where did the missing 0.5 bets go? To your opponent, because when you call in O8 to win half the pot, half of every bet you put in goes to the guy who wins the other half of the pot.
Key Principle: When playing for half the pot, you can't just multiple the pot by 0.5 and use those for your pot odds - your odds are actually worse than that. In fact, a good rule of thumb is to multiply by 0.4 when going for just half the pot.
The other key point that most people fail to account for is Redraws.
A redraw is where you can make your hand, but an opponent can make a better hand on the river card.
An example might be where you have 234K and your opponent has 66KT. The flop comes 569. He starts betting aggressively and so does another very tight player. So you know you are beat now for high and low. You count your outs here: any 2,3,4 or 7 are outs for you to make your straight. That is 13 cards (16 cards minus the 3 you have in your hand). Unfortunately, if you make your straight, your opponent has a redraw to a full-house. He'll make his boat 25% of the time on the river card.
Therefore, you need to discount your 13 outs by the % chance your opponent has to redraw you. In this case, you "get to keep" only 75% of your outs, and since 13 x 0.75 = 9.75, you need to think of your hand as having 9.75 effective outs, or outs to have the winning hand at the river.
This same principle applies to low hands too - it's called getting counterfeited. If you have A2TT on a 348 flop, then you have the nut low and have made your hand. But you need to dodge an A or 2 on the turn and river to keep your low. Here, you've made your hand already so you are avoiding the redraws. So you could think of your hand has having 39 effective outs to keep the nuts (45 unseen cards on the flop - 6 outs for the 3 aces and 3 2's).