Oyez, which is often pronounced as "Oh Yes," means "hear ye" in mediaeval French. the language of English governance until the fourteenth century.
It was traditionally used as an exclamation to attract everyone's attention to the beginning of a legal proceeding or the issuance of an official proclamation. In courtrooms, the bailiff would shout this out.
In modern times, courtroom proceedings are typically initiated with an exclamation by the bailiff or deputy clerk of "all rise" as the judge enters the courtroom, although proceedings at the United States Supreme Court open with "oyez" repeated three times.