Groat v. O'Hare was a United States Supreme Court case that dealt with the issue of whether a state court could enforce a contract that was made in violation of a state statute. The case involved a contract between two parties, Groat and O'Hare, in which Groat agreed to pay O'Hare a certain amount of money for the sale of a piece of property. The contract was made in violation of a state statute that prohibited the sale of property without a license. The Supreme Court held that the state court could not enforce the contract because it was made in violation of the state statute. The Court reasoned that the state statute was a valid exercise of the state's police power and that the state had the right to protect its citizens from contracts that were made in violation of the law. The Court also noted that the contract was not a valid contract because it was made in violation of the law. The Court's decision in Groat v. O'Hare established that a state court cannot enforce a contract that was made in violation of a state statute. This decision has been cited in numerous cases since then and has been used to support the idea that a state court cannot enforce a contract that is made in violation of the law.

Justice Harlan delivered a dissenting opinion in Groat v. O'Hare, arguing that the majority's decision was contrary to established precedent and would lead to an unjust result. He argued that the court should have followed prior decisions which held that when a contract is made with reference to certain rules or regulations, those rules become part of the contract itself and are binding on both parties. In this case, he noted that there were specific provisions in the lease agreement between Groat and O'Hare regarding how much rent could be charged for additional buildings constructed by either party during their tenancy. The majority had disregarded these provisions as being too uncertain or indefinite; however, Justice Harlan disagreed with this conclusion since it was clear from reading them what each party intended at the time they entered into their agreement. Furthermore, he argued that if such contracts were not enforced then landlords would be able to take advantage of tenants who build improvements on leased land without any legal protection against unreasonable increases in rent payments due to those improvements.