I think Nicolas has hit the nail on the head here.

At each session of the United States Supreme Court, as the Justices file into the Chamber, the Marshall intones thus: "The Honorable, the Chief Justice and the Associate Justices of the Supreme Court of the United States. Oyez! Oyez! Oyez! All persons having business before the Honorable, the Supreme Court of the United States, are admonished to draw near and give their attention, for the Court is now sitting. God Save the United States and this Honorable Court!"

Black's Law Dictionary states (I find the last four words amusing): OYEZ. Hear ye. A word used in courts by this public crier to commaned attention when a proclamation is about to be made. Usually pronounced "O yes."