Lying by omission is a deliberate choice to hide or withhold some relevant information that would influence the decision process of another party. In other words: "I know you would probably take another decision if I told you what I know, but I won't tell you".
Some argue that by not communicating false information, one is indeed not lying. However, the consequences of lying by omission can be as dramatic as lying for the other party. I don't see this argument as being valid to make such a practice an ethical one (I would like to avoid semantic debates about the definition of lying).
In addition, some people claim that by explicitly disclosing that they may lie by omission, that is, warn the other party in advance, is enough to remain ethical. To me, this is akin to saying: "It's ok if I lie to you, because I warned you". I don't see how this can be considered ethical.
Is there any way I can fully trust someone who claims that lying by omission is acceptable? If that person is in an authority position, with responsibilities, is there any way they can justify this?