This cry rang around the US recently when, for the first time, a session of the Supreme Court was broadcast on the same day the hearing took place. It may be heard at the start of proceedings in courtrooms, but otherwise is now mostly the preserve of elaborately costumed town criers in the more heritage-minded of English towns who wish to call attention to their pronouncements.
In Britain it’s often spelled and shouted oyes, and at times people think that is what is being said (Richard Barham borrowed that in his Ingoldsby Legends in 1840: “But when the Crier cried, ‘O Yes!’ the people cried, ‘O No!’”, though the joke was probably a wizened ancient even then, and it doesn’t work for Americans, who say “oh yay!” instead). It’s actually Norman French: oiez or oyez, the imperative plural of the verb oir, to hear.
The verb turns up also in a slightly different spelling in the term oyer and terminer, a commission once issued to English judges on a circuit to hold courts, literally “hear and determine”; this phrase is almost obsolete, though in Pennsylvania the county criminal courts are known by it. What is actually being said when oyez is shouted is “Hear ye!” or, in modern English, “Pay attention, you lot”, or “Listen up, you guys”.