fos

it could mean also :

Free Open Source
fosdir.com

mean

Free Open Source Directory
by peter edin October 16, 2008
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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
by peter edin October 16, 2008
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