See also Pot-Limit Omaha: Key Concepts
Player tendencies are an important concept in any poker game. Due to the small differences in equity between PLO hands, this concept is more valuable than in NLHE. In NLHE, sometimes you cannot fold due to the absolute strength of a hand. For example, if you re-raise with AA pre-flop and the flop is not something ridiculous such as KQJ monotone or 9TJ, it is probably correct to never fold if stack sizes are 100BB and below.
In PLO, knowing a player's tendencies can swing your marginal holdings from a call to fold or vice versa. Obviously, this concept applies to any poker game but it is more important in PLO because you are playing for stacks so often that small edges add up quick. On a flop of 963, it is standard to bet-call with holdings such as AAxx and AKQ9 against aggressive opponents who can raise your flop bet with a wide range; it is also standard to bet-call on two-tone flops such as 963 or T64 as well. However, if your opponent is tight and passive, you will burn money calling off all your chips here. You are almost always behind when the money goes in. If you are ahead, your opponent will have at least 45% equity in the pot. On a board of 963 with you holding AAxx, a hand as weak as 79TQ has 48.29% equity.
The key factor when playing these hands is to recognize how light your opponent is playing against you in re-raised pots. If your opponent is capable of shoving with any 9xxx or Txxx hands, or shoving with weak naked flush draws such as king or queen, you should bet-call. If you have never seen your opponent raise with 9xxx or Txxx or low flush draws, it is best to give him credit. Obviously, if his aggression factor against you is high in re-raised pots, then you should bet-call. If you lose, at least you will have some information on what types of holdings he is raising your flop bet with and you can adjust from there.
Before you think it is too hard to categorize different groups of players, let us tell you that such isn't the case in the small stakes PLO and mid stakes PLO games. There are usually two groups of PLO players: one group doesn't mind the variance and plays the game with a very loose aggressive style; the other dislikes variance and will only get it in with the nuts and rarely has a marginal holding in a big pot. There aren't a lot of players who are in between.
Obviously, such guidelines are simplified but it should give you a good starting point for developing reads on your opponents. As you gain more history with your opponents, you can add specific reads to their games.