In Spanish, when you buy some types of complementary articles of clothing you say that article has no size (sin talla). This can be shortened as "s/t"; a well-known abbreviation in Spanish (at least in Spain).
Is there some well-known and proper abbreviation for "no size" in English, or any other related term, more specific than "one size"? and if not, what is the correct abbreviation for "one size"?, "o/s", "O/S", "o.s.", "OS"?