O'Reilly v. Edrington was a case heard by the United States Supreme Court in 1891. The case involved a dispute between two parties over the ownership of a piece of land in the state of Texas. The plaintiff, O'Reilly, claimed that he had purchased the land from the defendant, Edrington, in 1876. Edrington, however, argued that the sale was invalid because he had not received full payment for the land. The Supreme Court ultimately sided with O'Reilly, ruling that the sale was valid and that O'Reilly was the rightful owner of the land. The Court reasoned that Edrington had accepted partial payment for the land and had not taken any action to reclaim it for over fifteen years. As such, the Court held that Edrington had effectively waived his right to reclaim the land and that O'Reilly was the rightful owner. The Court's decision in O'Reilly v. Edrington established an important precedent in property law, as it established that a seller who accepts partial payment for a piece of land and does not take any action to reclaim it for a period of time may be deemed to have waived their right to reclaim the land. This precedent has been cited in numerous subsequent cases and has been used to determine the ownership of land in many states.

In the case of O'Reilly v. Edrington, the Supreme Court was asked to decide whether a contract between two parties could be enforced if it had been made in violation of a state statute. The majority opinion held that such contracts were not enforceable and thus granted judgment for the defendant. Justice Field dissented from this decision, arguing that while statutes should generally be respected, they are not absolute laws and can sometimes be overridden by other considerations such as equity or public policy. He argued that in this particular case there was no evidence to suggest any harm would come from enforcing the contract and therefore it should have been upheld on equitable grounds even though it violated a state statute. In conclusion, he argued that courts must consider all relevant factors when deciding cases like these rather than simply relying on statutory law alone.