Cooley v. O'Connor was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The case arose when a prisoner, John Cooley, was held in a federal prison in Michigan. Cooley sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect this right. The Court's decision in Cooley v. O'Connor established that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. This decision has been cited in numerous cases since then, and it remains an important precedent in the area of federal-state relations.

In Cooley v. O'Connor, the Supreme Court was asked to decide whether a state legislature had the power to limit or control the authority of its courts in granting writs of habeas corpus. The majority opinion held that such limitations were unconstitutional and violated separation of powers principles. However, Justice Field dissented from this ruling, arguing that states have an inherent right to regulate their own judicial systems as they see fit and should not be subject to interference by federal courts. He argued further that Congress has no constitutional authority over state court proceedings and thus could not interfere with them either directly or indirectly through legislation like the Habeas Corpus Act of 1867 which was at issue in this case. In conclusion, he believed it would be wrong for federal courts to impose restrictions on how states manage their internal affairs when there is no clear violation of any law or constitutional provision involved.