os

simply its mean for most of us:

Open Source

may be the same characters have other meaning , but the most popular is open source.

And these is some other meaning for OS characters:
http://en.wikipedia.org/wiki/Os

open source directory

Mean

os dir

by peter edin October 16, 2008

fos

it could mean also :

Free Open Source

fosdir.com

mean

Free Open Source Directory

by peter edin October 16, 2008