Cred că vine de la "octavo".

OCTAVO
A volume measuring one-eighth the size of a full sheet of writing material, in which the QUIRES are usually produced by folding a sheet in half, in half again, and in half once more. **It may be abbreviated to 8° or 8vo.**
http://www.bl.uk/catalogues/illuminatedmanuscripts/GlossO.as...

Octavo (usually abbreviated 8vo, 8°, 8o, or Oct.) is a book size resulting from the use of standard size sheets of paper folded three times to make eight leaves.
http://en.wikipedia.org/wiki/Octavo_(book)

"JENNER, EDWARD (1749-1823).
Autograph Manuscript Signed, **8vo**, n.p., ca. 1822."
http://www.ilab.org/db/detail.php?lang=ru&membernr=1721&orde...

"The clergyman's instructor ... [Edited by John Randolph.] Fifth edition. / Randolph, John, successively Bishop of Oxford, of Bangor, and of London ; Clergyman.-- pp. vi, 443 ; **8o**..-- Oxford : at the University Press,
1843.
Held by: British Library"
http://66.102.9.104/search?q=cache:Hxq6-M-WvREJ:sas-space.sa...

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Note added at 2 days11 hrs (2008-06-15 19:24:39 GMT)
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Iată mai jos o definiţie de "in-octavo" în română, cu abrevierea respectivă:

in-octávo adj.; abr. in 8 o

IN-OCTÁVO adj. invar. (Despre formatul unei cărţi) În care coala de hârtie este îndoită în opt, formând 16 pagini; (despre cărţi, adesea substantivat) care are acest format. – Loc. lat.

http://dexonline.ro/search.php?cuv=octavo