Ya

Particle

Ya wato O da Turanci <> equivalent of O in English. The English vocative particle, used for direct address.
Ya ku waɗanda suka yi imani! idan anyi kira zuwa ga salla a ranar Jumu'a, sai ku yi aiki zuwa ga ambaton Allah, kuma ku bar ciniki. wancan ɗinku ne mafi alheri a gare ku idan kun kasance kuna sani.
Ya ku wadanda suka yi imani, idan anyi kira zuwa ga sallah a ranar Jumu'ah, sai ku yi sauri zuwa ga ambaton Allah, ku bar dukkan harkokin kasuwanci. wannan ita ce mafi alheri a gare ku idan da kun sani.
O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew.
O you who believe! When the call is made for prayer on Congregation Day, hasten to the remembrance of God, and drop all business. That is better for you, if you only knew. --Qur'an 62:9

The word O is always written in upper case in modern usage.
O is often used in translations from languages which have the vocative case.
Although it is not strictly archaic, the particle is sometimes used archaizingly. It conveys a formal or reverential tone.