Why words have an o instead of a u; woman vs. women

I. See “minim“… medieval cursive made it hard to read words with those letters close together. So respelled them with an O. Even though pronounced with a u or whatnot.  Example: 

Lunden/Lundon turned into London.

• London
• Come
• Love
• Some
• Won
• Son
• Tongue
• Dove
• Wonder
• Money

See yt guy “humanteneleven” “  why is son spelled with an o”

Also see https://londonist.com/2014/01/how-london-got-its-name

See Quote from wikipedia -

Etymology of London - Wikipedia

Later examples are mostly Lundon- or London-, and all the Anglo-Saxon examples have Lunden- with various terminations. He observes that the modern spelling with <o> derives from a medieval writing habit of avoiding <u> between letters composed of minims. The earliest written mention of London

Minims to the extreme. 

II. Woman vs. Women

Wif + Mann = female + human in old language. 

Wif + Menn = female + plural human 

Eventually, it all changed to sound like (wimman), spelled wiman. B/c supposedly/maybe(maybe not b/c why would we still pronounce it "wimmen" for the plural) easier to say Woh or Wuh than Wih, ppl pronounced it "woh" and then it changed to be spelled woman. Then they retroactively changed the spelling of wimen (wimmen) plural to women to match the woman spelling.  

See https://www.reddit.com/r/explainlikeimfive/comments/2zhkmh/eli5_how_come_when_saying_woman_and_women_we/

Originally it was wifman (wife-man, though man in its original meaning was gender nuetral) the plural was wifmen. Over time the words became wiman and wimen. In the Middle English period vowels underwent a shift and wiman became woman under the rounding influence of the ''w,'' while the plural retained the original vowel. Wiman>> woman. Wimen>> wimen.

Spelling was regularized much later, with the advent of the printing press. The spelling of the word is based off of the sigular, with the rounded vowel. Thus woman and women.

More complex answer for you.

<(-'○'-)>

Edit: mmk so as sacundim pointed out the vowel shift is a questionalble explaination for wiman to woman. I may have overstepped. I know there was a vowel shift around that time so I added it in to the other stuff I gleaned from the webernet. Wiman did shift to woman in that period, and etymology online says this is because of the initial w. But yeah. I was stretching on the vowel shift. Sorry. I'm a hobbyist not an expert. 

See comment:

Well, I'm no historical linguist, but I do have a linguistics degree, and I'm suspicious of this explanation.

Basically, you're saying that the sound change that changed the vowel in woman did not apply to women because the latter is plural. But the problem is that sound change doesn't generally work like that (my boldface):

Sound change is usually assumed to be regular, which means that it is expected to apply mechanically whenever its structural conditions are met, irrespective of any non-phonological factors (such as the meaning of the words affected).

Singular vs. plural is a non-phonological factor, so if the vowel change is conditioned by the [w] sound, then it should apply in both cases. It would be exceptional if the singular vs. plural context truly did this, so at the very least it needs a more careful explanation.

But then comment to counter the counter:

Yeah no that's really not the state of the art anymore. These are the Neogrammarian principles of sound change. They describe an idealised form of sound change, but in reality it doesn't really work like that. Lexically or morphologically conditioned sound change is quite common.

Just a small example: Take the word-initial voicing of English /θ/. It only happened in a small group of words.

EDIT: You also left out the following sentence in your quote:

On the other hand, sound changes can sometimes be sporadic, affecting only one particular word or a few words, without any seeming regularity.