Oil on steel
Oil on steel is an Opera theme that features an abstract image of an old locomotive.

Octopi
Octopi is an open source and powerful application designed to act as a front-end to the Pacman package manager, using the Qt libraries.

OpenMandriva
Mandriva Linux is a modern Linux operating system that combines modern technologies with an attractive user interface, based on KDE.

o42a
o42a is an open source, high-level and general purpose programming language software that can be described as compiled, statically-typed, prototype-based,  logic-driven, and primarily declarative.

Orthanc
Orthanc is an open source, simple, lightweight, yet powerful RESTful standalone DICOM server for health care and medical research.

OpenTBS - Skrol 29
OpenTBS is an open source plugin for the TinyButStrong template engine, allowing users to easily create OpenOffice and MS Office documents.

O_O_O
O_O_O is an Opera theme designed especially for serious fans of Opera Software. It features the Opera logo.

Overclockix
Overclockix is an open source Linux operating system based on Debian GNU/Linux and designed especially for computer enthusiasts and overclockers.

OnlyTab
OnlyTab is an Opera extension that will reduce memory consumption and clutter by converting all opened tabs into a single tab, which contains a list ...

Orta Cinnamon Light
Orta Cinnamon Light is an open source theme for the Cinnamon desktop environment, a port of the Orta GNOME Shell created by Half-Left.

Open Link In Pinned Tab
open-link-in-pinned-tab is a Firefox extension that implements an entry in the right-click context menu, allowing users to easily open a link into a pinned tab.

OS4 OpenDesktop KDE
OS4 is an open source and custom Linux distribution based upon the Ubuntu OS. It aims for ease of use and user friendliness and falls ...

openSUSE 12.3 skin
openSUSE 12.3 skin is an Opera theme designed for all fans of the openSUSE operating system, especially for the openSUSE 12.3 distribution.

Online Chess Games
Online Chess Games is a Firefox extension that add a button on the Add-On bar, allowing users to play the following chess games: Knight Switch, ...

OSX Like
OSX Like is a free theme for the Cinnamon desktop environment that makes it look just like the Mac OS X desktop. It contains:

Open eCard Legacy Activator
Open eCard Legacy Activator is a Firefox extension that runs in the background and scans eCard object tags.

Online Jigsaw Games
Online Jigsaw Games is a Firefox extension that adds a button on the Add-On bar, allowing users to play Jigsaw games, such as Jumping Frog, ...

Old Add Bookmark Behavior - White Alice0775
Old Add Bookmark Behavior is a Firefox extension that makes "Bookmark This Page", "Bookmark This Tab" and "Bookmark All Tabs" behavior equal with Firefox 2.

One Direction
One Direction is an Opera theme designed especially for all fans of the One Direction boy band.

OPML Support - Christopher Finke
OPML is a file type that is widely used to distribute lists of RSSewsfeeds.

Old Comic Interactive Persona
Old Comic Interactive Persona is an awesome looking Firefox theme especially designed for comics fans. It features a multimedia sidebar for the latest news.

OutlookAlike
OutlookAlike is a Thunderbird extension that will make the Thunderbird email client look like the Microsoft Outlook application.

Open Tennis for Opera
Open Tennis is an Opera extension that lets users play a nice and fun tennis game, via the awesomewebgame.com website.

Office Interactive Persona
Office Interactive Persona is a Firefox theme specially designed for serious fans of the Office TV show. It includes a boss button, to look like you're ...