Absence of vowel combination /ou/ in Spanish

Question

Spanish has many words containing the diphthongs /au/, /eu/ and /iu/, but the only instances of words containing /ou/ (as a diphthong or in hiatus) are a very small set of foreign loanwords:

bou, proustiano, soul, noúmeno

or compound words of the form ...o + u...:

estadounidense, estadounidismo, genitourinario, finoúgrio

The neighbouring romance languages Portuguese and Catalan both have a large number of words containing /ou/, is there a specific reason why Spanish doesn't?

There even seems to be a tendency to convert loan /ou/ to /o/, as in the Spanishised name of the Galician city Ourense (Orense), and its associated demonyms.

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^{(My immediate thought was that all instances of Latin /ou/ became /oβ/ ~ /ob/ in Spanish, but this seems to be contradicted by the fact that /au/ and /eu/ remained in many words.)}

Answer 1

It seems this was a combination of:

1. 'ou' being rare in Latin words and only in environments where vowels would undergo changes in the evolution to Spanish, and
2. instances of vowel + consonant transmuting into vowel + u/w in the evolution from Spanish to Latin being lost with back vowels (like o):

8. Adjustments due to vowel syncope

...

Finally, syllable-final /b/ (realized as [β]) was generally semivocalized to [w], although this latter sound was later lost through assimilation if it followed a back vowel (see the codo example below):

• dēb(i)ta → [ˈdeβða] debda → [ˈdewða] deuda ‘debt’
• cap(i)tālem → [kaβˈðal] cabdal → [kawˈðal] caudal ‘money, river flow’
• cŭb(i)tum → [ˈkoβðo] cobdo → [ˈkowðo] → [ˈkoðo] codo ‘elbow’

Also, absentia → ausencia, baptista → bautista, rapĭdus → raudo etc

So, the environments where /ow/ ~ /o.u/ existed in the past/exist in other romance cognates but don't occur in Spanish are:

1. a) /o/ monothong stressed, /o/ diphthongized to /we/

   ovum → huevo
   bovis → buey
   novus → nuevo

   b) /o/ monothong unstressed, /w/ → /β/

   bovīnus → bovino

2. a) /oβ/ → /ow/ diphthong, assimilates to /o/

   [ˈkoβðo] → [ˈkowðo] → [ˈkoðo]

   b) /ob/ preserved

   obcaecāre → obcecar

Answer 2

Two things:

1- The words you have listed are not Spanish, but loans, adaptations from ancient words or neologisms based on modern languages, foreign surnames, etc. 2- Ourense was never a loan into Spanish. As a matter of fact, the artificial revival of Galician forced a Galician pronunciation in places which had already lost it. Having lived in multilingual Spain for many years in my childhood (the real multilingual areas, not what the propaganda and official story tells you), I can tell you noone said Ourense or A coruña back then, at a time in which you could hear Girona, Xixón por other place names. Spanish never reduced Ou to O in Ourense. That has nothing to do with, say, the reanalysos of (O) Porto as Oporto.

2-Yes, there is a specific reason: it is called monophthongisation in the phonological diachrony of the language. As simple as that. Or you can see it as an issogloss in dialectological terms.