Setting
The setting of the OS-tan universe is what joins all the OS-tans and their factions together. But what the setting looks like and how it works is subject to much interpretation as none of these are set in stone.
The setting from known OS-tan 'canon'
The canon doesn't go into very much detail about the setting, simply focusing on the characters first and foremost. Sometimes the setting isn't specified. Occasionally other characters appear, such as Toshiaki (representative of the user) and Bill Gates.
Many of the Japanese OS-tan comics seem to take place in an unspecified part of Japan, given the OS-tan phenomenon's Japanese origins. Most of the comics star the Windows-tans, and as such take place mainly in the Windows Family household, which itself is portrayed as a traditional Japanese-style mansion, which is also shown in many fanart.
It's assumed that since the Windows and Macs are rivaling families, that in-canon they live near each other.
Not much is given in-canon about the where the Mac-tans live, aside from the Mac Manga, which shows them (or at least Mac OS 9-tan, Mac OSX-tan and the OSX-kuns) living in a large western-style villa.
OS-tan expanded universes
While the Japanese-made fanon usually takes place on a small scale (i.e: In a single house with Toshiaki as the master), The expanded universe which makes up western fanon often takes place on a large scale.
Fantasy Cyberpunk Culture Theory
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In this interpretation, the OS-tan universe doesn't take place in the real world, but a fantasy cyberpunk world that may be partly based off of it. Real people usually don't appear, and the OS-tanverse is comprised of solely of the OS-tans at the top of the hierarchy, with hardware-tans, program-tans and file-tans at their service. There is no map of this fictional world, but each faction (except for the CIOST and wanderers who live around the world) has its own territory, led by an OS dynasty.
The great number of anachronisms, of technology and fashion are explained by stylistic choices of individual OS-tan cultures. In this theory, fashions of historical OS-tans are also symbolic of relative technological advancement.
Since this was for some time the most popular interpretation, it set up the guideline that older OS or computer-tans usually wear older fashions. Even in canon, there are some traces of this, with 95-tan wearing a kimono, which was common college attire for women in the Meiji era of Japan, and 3.1-tan wears western-style attire of the same time frame. There is a lot of flexibility to this guideline, such as the late 70's and early 80's home computer-tans wearing different time periods of fashion, but are more old-fashioned than 3.1 and 95. There are also some exceptions, in canon, there is MS-DOS-tan's modern schoolgirl attire.
This used to be the main theory used in OS-tan expanded universe works, but is now second to the real-world based theory. However, the two aren't necessarily incompatible.
Real World-based Theory
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This theory, which has gained a lot more favor in the western fanon, involves real users and real locations in what looks like the real world with some futuristic cyberpunk elements. The inclusion of real people of the companies who created the computer systems serves as a way to neatly resolve the origins of the OS-tans in-universe as having been artificially created.