Omega stands twenty fourth, writes the vowel o, and carries the numeral 800. It is Greek omega, taken over whole, and it is one of the letters that exist in Cyrillic because Greek had them rather than because Slavonic needed them.
Slavonic has one o. Cyrillic gave it two letters, on and omega, and no rule of pronunciation could ever distinguish them. What developed instead was a distribution by convention, and the conventions grew more elaborate rather than less. In the printed church books omega is written at the beginning of certain words, in certain prepositions and interjections, and in a small set of fixed forms, and the printers added further variants: a broad form, and a decorated form used in exclamations.
Its numeral value of 800 is what made it indispensable. The hundreds series needs nine letters, and removing omega would have left the series short with nothing available to replace it, so the letter was protected by arithmetic for as long as the arithmetic was in use.
It was among the letters struck from secular printing when a civil type was introduced in the early eighteenth century, and it has no place in any modern Cyrillic alphabet. In the church books it is still set and still read, which is the pattern for most of the letters this alphabet has lost: withdrawn from ordinary use, retained in liturgy.