o.s.abbr.1. Latin oculus sinister (left eye)2. old series3. or o/s out of stockold stylen.1. Printing A style of type originating in the 1700s and characterized by slight contrast between light and heavy strokes and slanting serifs.2. Old Style Abbr. OS or O/S The method of reckoning dates according to the Julian calendar.os 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.]OsThe symbol for osmium.OSabbr.1. or O/S Old Style2. operating system3. ordinary seamanAmerican 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.