OCTAVO is a 6 letter word that starts with O.

1. \Oc*ta"vo\, n.;pl. {Octavos}. [L. in octavo; in in +
   octavo, abl. of octavus. See {Octave}.]
   A book composed of sheets each of which is folded into eight
   leaves; hence, indicating more or less definitely a size of
   book so made; -- usually written 8vo or 8[deg].
   
   

2. \Oc*ta"vo\, a.
   Having eight leaves to a sheet; as, an octavo form, book,
   leaf, size, etc.