United States v. O'Grady was a United States Supreme Court case that dealt with the issue of whether a person could be convicted of a crime if the act was not specifically prohibited by law. The case involved a man named O'Grady who was charged with the crime of "malicious mischief" for cutting down a tree on public land. The government argued that O'Grady should be convicted because the act of cutting down the tree was a violation of the public trust and was therefore a crime. The Supreme Court disagreed with the government's argument and held that O'Grady could not be convicted of a crime unless the act was specifically prohibited by law. The Court reasoned that the government could not punish someone for an act that was not specifically prohibited by law. The Court also noted that the government could not punish someone for an act that was not specifically prohibited by law even if the act was considered to be wrong or immoral. The Court's decision in United States v. O'Grady established the principle that a person cannot be convicted of a crime unless the act is specifically prohibited by law. This principle has been applied in many subsequent cases and is an important part of the American legal system.

Justice Field delivered the dissenting opinion in United States v. O'Grady, arguing that the majority's decision was inconsistent with prior Supreme Court precedent and would lead to an unjust result. He argued that Congress had not intended for a person who committed a crime before 1866 to be subject to prosecution under the Revised Statutes of 1874, which were enacted after his alleged offense occurred. Furthermore, he noted that if such prosecutions were allowed it could have serious implications on individuals’ rights as they may be prosecuted for acts done years ago without any warning or knowledge of their potential criminal liability at the time they acted. Justice Field concluded by stating that allowing this type of prosecution would violate fundamental principles of justice and fairness and should therefore not be permitted by the court.