On stands sixteenth, writes the vowel o, and carries the numeral 70 from Greek omicron. Its name is the Slavonic third person pronoun, the word for he.
Its numeral value shows how far the alphabetical order and the number series have parted company by this point: the sixteenth letter carries the seventieth number, because six letters supplied for Slavonic sounds have been inserted into the sequence without taking numbers with them. A reader who tries to count position from value, or value from position, will be wrong from here to the end of the alphabet.
The letter is one of a pair. Cyrillic has two signs for o, this one and the omega taken from Greek, and neither can be predicted from the sound. Which is written where is a matter of orthographic convention, and the conventions differ between the manuscript tradition and the printed church books.
The church printing tradition multiplied the forms further. Slavonic type carries a broad or wide o used at the beginning of certain words, and a doubled form in a small set of exclamations, so a printed liturgical page can show several distinct signs all saying the same vowel. That proliferation is a printers inheritance rather than a scribal one, and it is one of the reasons Church Slavonic typography is harder to set than the alphabet alone would suggest.