The United States Supreme Court case of The United States v. Antonio Maria Osio concerned the issue of whether a foreign-born individual who had resided in the U.S. for more than twenty years, but had never been naturalized, was eligible to own land under Spanish law as it existed prior to cession by Spain to the U.S.. In this case, Osio argued that he was entitled to ownership rights over certain lands in Florida based on his long residence and occupation of them before they were ceded from Spain to the United States following its victory in the Mexican War (1846–48). The court held that although Osio's claim could not be recognized under American law due to his lack of citizenship status at time when title passed from Spain, it could be recognized under Spanish law which did not require naturalization or other formalities for foreigners residing within its territories for an extended period of time. Thus, despite being ineligible for recognition as a citizen or property owner according to American laws governing such matters at that time, Osio's claim was upheld by virtue of existing Spanish legal principles applicable during pre-cession times which allowed him access and protection with respect thereto regardless of nationality status

In the case of The United States v. Antonio Maria Osio, the Supreme Court was asked to decide whether a person who had been born in Spain and resided there for many years before coming to the United States could be considered an American citizen. The majority opinion held that since Osio had not taken any steps to become a naturalized citizen, he did not have citizenship rights under U.S law and thus could not bring suit against the government in federal court. However, Justice Grier dissented from this decision arguing that although Osio may never have formally declared his intention to become an American citizen, he should still be allowed access to federal courts as if he were one due to his long-term residence in America and his loyalty towards it during times of war with Spain. He further argued that denying him such access would violate both common sense principles of justice as well as fundamental constitutional guarantees regarding equal protection under the law regardless of race or national origin.