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.
(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.)