Cyrillic on writes the same vowel and takes the numeral seventy, the value of Greek omicron, against the eighty that Glagolitic reaches by its own counting. The gap is ten at this letter, having been twenty at the letter before, because the numbering of both systems has just crossed from the tens into a different stretch of the order at slightly different points.
Both alphabets carry two signs for the rounded back vowel and both took the redundancy from Greek, which had two letters for a sound that by the ninth century had become one. In both scripts the second sign, far down the order, is the one used in Greek loanwords and in certain fixed spellings, and in neither does it record a distinct Slavonic sound.
The letter is also a reminder that the two scripts were in use at the same time and in the same places for a period. Manuscripts survive in which Glagolitic writing has been scraped away and Cyrillic written over it, and the recovery of such under-texts by imaging is one of the few routes by which new early Glagolitic material still comes to light. What that overwriting shows is a change of scribal practice within one tradition, not the arrival of a foreign script.
The names of the two letters differ by an ending, onu beside on, which is the ordinary sort of variation between the abecedaria. Both continue the same Slavonic demonstrative, and both alphabets recite the letter in the same place. The atlas pairs them as counterparts of sound, name and position, which is a claim about correspondence and not about descent.