Multiocular O - just going to quote Wikipedia:

Multiocular O (ꙮ) is an exotic glyph variant of the Cyrillic letter O. This glyph variant can be found in certain manuscripts in the Old Church Slavonic phrase "серафими многоꙮчитїи" (serafimi mnogoočitii, "many-eyed seraphim").

There are apparently other instances of Old Church Slavonic scribes playing around thus, rather like people dotting their i-s with little hearts or me turning the letter S into a python given the slightest excuse. 

Hat tip: Odious.