Like all theories pertaining to the OS-tan universe, the views presented in this article may not be considered canon or even fanon by all people. The majority of these theories come from various character-creation threads on OSC, as well as the personal views of various OS-tan artists and writers. The goal is to gather all theories at a central location before putting them in their proper articles (under the Theories and Notes section).
- 1 Theories about the wider OS-tan universe
- 2 Theories about present and past factions
- 3 Theories about non-factional groups
- 4 Theories about families
- 5 Theories about individual characters
Theories about the wider OS-tan universe
OS-tans and ethnicity
While the OS-tan universe is populated by characters possessing a wide range of body types, skin colors and styles of dress, the topic of character ethnicity has rarely been broached, either in-canon or theory. This lack of attention to race is somewhat justified by the nature of OS-tans themselves - since they are not human beings, they do not belong to human races per se. At the same time, OS-tans are frequently depicted as having bodies that are nearly indistinguishable from "normal" human bodies - it only makes sense that they would display the same level of variability seen in humans.
There is a fanon precedent for ethnic variability in OS-tans; they are often depicted as belonging to whatever ethnic group is most common in their country or region of origin - for instance, personifications of Japanese systems are typically ethnically-Japanese. Furthermore, many OS-tans have been explicitly stated, by their creators, as belonging to a certain ethnic group - for instance, PCLinuxOS-tan is Mexican-American, TRS-80-tan is portrayed as Hispanic, Pardus-tan is Turkish and some versions of Ubuntu-tan depict her as being of native South African heritage.
Other times, ethnicity is only hinted at by the creator or depicted in drawn form, with little attention being given to the character's ethnicity in written depictions. For example, this may be the case with CTSS-tan and MCP-tan, who have been suggested by their creator as being Chinese- and African-American, respectively.
Humans and OS-tan magic
It has been theorized that OS-tans often have wide ranges of magical skill. In addition to their self-defense ("security") powers and other forms of sorcery used in their day-to-day lives ("personal powers"), they have a certain sorcery abilities that they use in whatever job they're assigned to - spells related to business stuff, keeping track of information, collecting data from scientific experiments, etc. Unlike their more reflexive security and personal-use powers, these "work spells" can be easily invoked by human users and program-tans via commands. 
These commands can take a variety of different forms. Some OS-tans - specifically, real-time, timesharing and interactive OS-tans - process these commands in real time, and need only for a user to request them to carry them out. Meanwhile, batch-processing OS-tans require the commands to be written down before they can process them, and the spellcasting process tends to be more ritualistic and prolonged than it is for interactive OS-tan spells. (Analogous to the spell scroll vs. regular casting process used in various RPGs.)
These ideas have implications for single-user and single-tasking vs. multiuser, multitasking and timesharing systems, since single-user OS-tans would presumably only be able to perform spell-casting for one person at a time, single-tasking OS-tans would only be able to perform single spells, while multitasking OS-tans could perform a variety in short succession and multiuser and timesharing OS-tans provide magical requests for large numbers of people. However, this subject has yet to be fully explored.
A sub-theory suggests computer storage mediums, such as punch cards and paper-tape, are used as spellbooks by OS-tans and humans.
Another theory proposes that, with enough time and training, humans have the potential to master computer-tan "code magic". However, this is regarded as a very rare occurrence, due to the difficulties posed by learning the intricacies of computer magic, as well as the biologically- and psychologically-foreign nature of code magic. 
Humans and codespace
User NejinOniwa's theories on human-codespace interaction. 
Operational vs. non-operational OS-tans
The working status of an OS or computer has a direct effect on its personification. Often times, when a computer/OS falls out of use, its OS-tan is considered deceased. But there is no widely-accepted classification scheme for those computers and OSes which are between extant and expired.
One theory proposes that there would be a variety of states an OS-tan can fall into: working (still being used for commercial purposes, may be current or non-current), working/hobbyist (systems that are working, but also out-of-date and used as hobbyist computers), hobbyist (no longer used commercially, but maintains a hobby userbase) and non-working, non-hobbyist and fully retired (no known userbase, but the system is still considered "alive").
Put into OS-tan terms, a working OS-tan would be just that - currently holding down some sort of employment or otherwise contributing to some commercial end. The uses for hobby systems are is harder to define, but it has been speculated that they are kept around as sort of living records of the past, studied by scholars wishing to learn from their archaic forms of magic and about the history they witnessed. They may also be used in contests, reenactments and for other forms of entertainment. Thus, in spite of having a low work value, they are still highly regarded for their historical and recreational purposes. OS-tans who are both working and hobbyist would be a combination of the first two, while non-working, non-hobby systems would be the true retirees of the OS-tan universe, contributing neither to the OS-tan "workforce" nor the field of recreational computing.
MCP-tan, PDP-8-tan, PDP-1-tan and Multics-tan are a good example of each of the respective categories. 
Theories about present and past factions
The DEC-tans became a military faction in response to outside forces
The DEC-tans did not originate as a military faction. In fact, their first decade or so was entirely peaceable, and the members opposed to fighting for the most part. However, in the late 1960s, the Data General faction was founded not far from DEC territory; unlike the DECs, the DG faction was an aggressive group, and often made direct threats to the DEC-tans. Fearing for their safety and security, the DEC-tans took up arms and taught themselves how to fight. 
Fear of the IBM-tan faction may have also contributed to this shift.
The Data General-tan Faction
Like the DEC-tans, the DG-tans had their own faction. It was founded in 1968, when PDP-8-tan's creator abandoned DEC after the higher-ups refused his plan to create an improved successor for her. The Data General faction was founded, with Nova-tan becoming the de-facto leader of their computer-tans.
The DG-tans were founded as a military faction and made constant threats to destroy their neighboring factions - in particular, the DEC-tans. They were a notoriously austere and authoritarian group, in stark contrast to the more liberal and playful DEC-tans, and may have possessed more advanced technologies. However, they were also very small, somewhat impoverished and suffered poor leadership, and could never overtake the far larger, more popular and better-organized DECs.
Theories about non-factional groups
Animal-featured OS-tans have specialized forms of magic
All OS-tans have some form of innate magical ability, but not all of them express it the same way. It has been speculated that OS-tans who have animal features - such as animal ears, wings, tails, horns, etc. - would possess and utilize forms of sorcery (henceforth referred to as "animal magic") that are not seen in OS-tans who have strictly humanoid features.
Under this theory, animal-featured OS-tans have specialized powers and modes of attack. These powers would be naturally stronger, subconsciously-rooted and easier to invoke than the magic of their non-animal-featured peers. Non-animal-featured OS-tans would have to undergo extensive training to become skilled at magic-wielding, but these OS-tans may be able to use more generalized forms of sorcery. These OS-tans can choose which forms of magic they want to learn; however, since they would have to use a conscious spell-casting process to invoke their magic, it is slower-acting than the magic in animal-type OS-tans.
Multics originated animal features/animal magic in most OS-tans
Multics was the first (documented) OS-tan with animal features. Her daughter Unix-tan sports owl ears and has wings, CB Unix has cardinal features, various BSD-tans have horns and tails (and one has dragonfly wings), Plan 9-tan has bunny features, the OSX-tans have cat features, Minix-tan has raccoon ears, various Linuces have animal features, etc, etc. OpenVME-tan is loosely related to both Unix AND Multics, and she has wings too.
One could even make a case that Multics passed down animal traits into the Windows NT family via OpenVMS, who shared a loose spiritual relation to Multics. While VMS-tan doesn't have any outward animal features, she it very closely linked with sharks, and seems to display a lot of shark-like personality traits. Her granddaughter, Inu-T-tan, has dog features.
That being said, there are several OS-tans that that have animal features despite being seemingly unrelated to the Multics line - for instance, KolibriOS, GS/OS and Bendix G-15-tan.
Theories about families
Early factions were wary of the Unices because of their political and magical power structure
Until the Unices came along, OS-tan clans/factions were typically comprised of groups loosely- or un-related OS-tans. There would sometimes be small family structures - with a mother and children, or a group of sisters - but it was very rare to see large families. Magical powers were usually an entirely individual, and often learned, skill.
The Unix "clan" was different - with a distinct lineage, all the Unix-tans being related by blood or cloning, and all/most of them deriving their magic from a shared "pool" of power. Animal-type OS-tans are common among the Unices, so it's probably not a stretch of the imagination to think that their distinct forms of animal-type magic and unusual physical features were strange, and sometimes upsetting, to outside OS-tans. Combined with their reputation for assimilating cultures, you can see why so many outside factions were afraid of them and wary of letting them join their groups.
Theories about individual characters
Multics-tan is CTSS' and BESYS' child
In an alternate theory, Multics-tan is the daughter of both CTSS and BESYS-tan. Evidence cited in support of this theory includes Multics' hair color, eye color and the fact that Multics was ordered by CTSS to live with BESYS in her youth (although she and BESYS mutually disliked each other and the stay was short-lived), as well as physical and mental similarities between BESYS and Multics' child, Unix. 
VMS-tan has powerful empathic skills
It's odd that the personification of an OS with exceptional networking features has always been portrayed as generally emotionally-cold and distant. It has been suggested that VMS-tan is actually well-attuned to the emotions of others, but that she has to isolate herself and/or carefully choose her friends because she is easily dragged down by the bad energy of others.
Plan 9-tan is a hardcore geek
She's a confirmed space- and science-geek who cosplays, loves B-movies, anime, and is a Touhou fan. How can she NOT be a huge nerd?
Unix-tan collaborated with the DEC Faction
In the late 1970s DEC released an in-house, lightly modified version of V7 Unix. Since Unix-tan represents all research Unix variants (V7 included), it stands to reason that Unix-tan collaborated with the DECs during this time.
Windows 3.1-tan is still working
Since Windows 3.1 has been reincarnated as an embedded OS, some theorize that 3.1-tan leads a double-life - she presents herself as retired to her family, but in reality still holds down work in the highly competitive embedded market.