If you start playing Omaha after having played Texas hold'em then it is very likely that you will at some point mistake your hand strength because of this rule. No, AAK8 on a 5679A board does not give you a straight nor is AAJ7 on a JTJ33 board a full-house.
Omaha is a game in which the winning hand often is the nuts, the best hand possible given the community cards. Therefore you should not be drawing to hands that are anything other than the nuts, because you will likely be drawing to a second best hand otherwise and lose a lot of chips.
With four hole cards every Omaha player essentially has six possible hold'em starting hands. Selecting your starting hands well means that you are looking for hands to play in which as many of those six combinations of two cards are good hold'em starting hands. All the four cards should be working together giving straight, nut flush and top set potential (higher pairs).
Continuing with the last tip about starting hand selection, some of the best Omaha (high) starting hands are shown above. Here 'ss' stands for double suited: two cards of the same suit for two different suits.
If you have the nuts on the flop (say, top set/three of a kind) and straight- and flushdraws are possibly out there then it makes your hand stronger if you have some of the cards that would make those draws yourself. These cards are called blockers: they block your opponents from hitting their draw as often as they would if you didn't have had any blockers at all.
Re-draws make your strong hands even stronger. Imagine that you and your opponent both flop the same nut straight, but you also have flopped a set and a flushdraw with it (giving you a re-draw to a full-house and a flush). Now your opponent is a big underdog to win the hand despite having flopped the nuts on the flop. In addition, if someone were to make an even higher straight on the turn, you could still beat this opponent on the river by making your re-draw.