In Catholic teaching an omission is a failure to do something one can and ought to do. If this happens deliberately and freely, it is considered a sin.
The degree of guilt incurred by an omission is measured, like that attaching to sins of commission, by the dignity of the virtue and the magnitude of the precept to which the omission is opposed, as well as the amount of deliberation.
A person may be guilty of a sin of omission if he fails to do something which he is able to do and which he ought to do because he has put himself into a state or situation whereby he is unable to complete the action. For example, if a person chooses to become inebriated and is therefore unable to perform a necessary task, that person is responsible for that failure, even though that person is physically unable to perform the task because he or she knowingly put themselves into a state (drunkenness) where accomplishing the task was impossible.
Paul the Apostle refers to this sin directly when he states "For I do not do the good I want ..." (Romans 7:19).
Testament / Billy, Peterson, Skolnick
The day's vanished in the night, soon the children pray
Pray themselves alone, the hate build from their pride
You can't take away, can't take what they own!
The eyes watching from the sky, guides them through the haze
Calling far away, the rain is black as black, black as night
Oh let them pray!
False sense of pride, satisfies
There's no reason for suicide
Use your mind, and hope to find
Find the meaning of existence...
The cry's answers wonder why, why it's time to die
Time to die alone, cause I'm wasting too much time
Wandering down the line reason why I go my own!
False sense of pride, satisfies
There's no reason for suicide
Use your mind, and hope to find