These were all originally Martin designations.
Maury has a great chart at this link:
http://www.maurysmusic.com/martin_by_body_size
To complicate things for the novice, the 0, 00, 000 and dreadnaught all were made at various times in both 12 fret and 14 fret versions, with different shaped bodies for each. So there are correctly speaking two 0 size/shapes, two 00, two 000 and two dread.
The OM by definition is 14 fret.
The M (for which 0000 means the same size shape) is only a 14 fretter.
There are further cloudings by Martin in the modern age of the 15 & 16 series, which have changed previous easy to remember spec designations, mostly relating to scale length and nut width.
Hope that helps.
