Shardt, Yuri; Simmons, Nikita; Andreev, Aleksandr (2011-02-25). Proposal to Encode Some Outstanding Early Cyrillic Characters in Unicode(PDF) (Report). Unicode Consortium. p. 1. L2/10-394R. Retrieved 2018-02-28. The crossed o is primarily used in the word окрест (around, in the region of) in early Slavonic manuscripts.
See, for example: Н. П. Саблина. Буквица славянская. Поэтическая история азбуки с азами церковнославянской грамоты. СПб.: Ижица, 2001. OCLC51079099ISBN978-5-9903415-6-2.