Omission is, in Catholic teaching, the failure to do something one can and ought to do. If this happens advertently and freely a sin is committed.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 by failing to do something which he is unable to do, by reason of a cause for which he is entirely responsible, as when a person knows that drinking to drunkness will incapacitate him, and yet drinks.St. Paul refers to this sin directly when he states "For I do not do the good I want..." (Romans 7:19).
Catholic Encyclopedia "Omission"
Religious ethics Christian theology