OsThe symbol for osmium.OSabbr.1. or O/S Old Style2. operating system3. ordinary seamanos 1 (?s)n. pl. o·ra (ôr??) A mouth or an opening.[Latin ?s, mouth; see ?s- in Indo-European roots.]os 2 (?s)n. pl. os·sa (?s??) A bone.[Latin, bone; see ost- in Indo-European roots.]os 3 (?s)n. pl. os·ar (??sär?) See esker.[Swedish ås, ridge, from Old Norse ?ss.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.os (?s) n, pl ossa (??s?) (Anatomy) anatomy the technical name for bone[C16: from Latin: bone; compare Greek osteon]os (?s) n, pl ora (???r?) (Anatomy) anatomy zoology a mouth or mouthlike part or opening[C18: from Latin]os (??s) n, pl osar (???s??) (Geological Science) another name for esker[C19 osar (pl), from Swedish ås (sing) ridge]Os the chemical symbol for (Elements & Compounds) osmium OS abbreviation for 1. (Education) Old School 2. (Physical Geography) Old Style (method of reckoning dates) 3. (Nautical Terms) Ordinary Seaman 4. (Surveying) (in Britain) Ordnance Survey 5. outsize 6. (Languages) Old Saxon (language) Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014os1 (?s) n., pl. os?sa (??s ?) a bone. [1540?50;